Borrow or Sell for the Down Payment? Box Spreads, Pledged-Asset Lines and the Vest Bridge

The closing date is in April. A large vest lands in May. The down payment needs $100,000 that will be sitting in cash six weeks too late. The portfolio could cover it today, but every lot in it carries a gain, and selling now means a tax bill on money you only needed for a month and a half.

Three answers come up. Sell the shares and take the tax. Borrow against the portfolio with a pledged-asset line or margin. Or sell a box spread on the S&P 500 index, which borrows near Treasury rates for a fixed term. Which one is cheapest depends on two tax rules most comparisons leave out, and on whether the gap really closes on a known date.

Three ways to cover the gap

Sell appreciated shares. The cash is clean and there is no loan to manage. The cost is the tax on the embedded gain, due with the next return. When the vest arrives you can buy the position back, and because the new basis is higher, part of that tax is only paid early rather than added. How much depends on when you would otherwise have sold. Lots you would have held for life lose the most, because the basis step-up at death would have erased the gain entirely.

Borrow on a pledged-asset line or margin. The shares stay invested and no gain is realized. The rate floats, the loan is callable, and a large brokerage’s schedule in August 2026 ran from about 6% to over 11% depending on the product and balance, as tabulated in our box spread guide. Interactive Brokers’ tiered margin sat near 5%.

Sell a short SPX box spread. Four index options at two strikes raise cash today against a fixed amount owed at expiration, whatever the index does. One 1,000-point box owes $100,000. Cboe’s box rate index for the December 2026 expiry read 4.24% on August 19, 2026.¹ The mechanics, the rate conventions, and the ways a box can go wrong are covered in Box Spread Loans.

The two tax rules that decide it

Margin and pledged-asset interest for a home gets no deduction

Interest deductibility follows the use of the borrowed money, not the collateral. Treasury Regulation 1.163-8T allocates debt by tracing where the proceeds go.² Money borrowed against a brokerage account and spent on a house is traced to a personal expenditure, so its interest is personal interest. It is not investment interest, because it did not buy an investment, and it is not mortgage interest, because IRS Publication 936 requires the debt to be secured by the home through a recorded instrument.³

A box loss is a capital loss, with a $3,000 limit against wages

SPX options are nonequity options under section 1256, so a box’s financing cost shows up as a capital loss, 60% long-term and 40% short-term, marked to market at year end.⁴ Nothing about that depends on what the cash bought. The loss offsets realized capital gains without limit. Against ordinary income, section 1211(b) allows only $3,000 of net capital loss a year, with the rest carried forward.⁵ On a three-month, $100,000 bridge the whole cost fits under $3,000, so the cap does not bind. On a multi-year box it does.

What each path costs

For one modeled buyer bridging $100,000 for three months, at the August 2026 rates above, a 20% federal capital gains rate plus the 3.8% net investment income tax, no state tax, a 35% ordinary rate, and lots that are 60% gain:

The ranking is stable across reasonable inputs as long as the lots carry a real gain and the bridge is short. It flips when the shares are close to basis: with 2% embedded gain, selling costs less than any loan. It also narrows when your broker’s margin is cheap. Against Interactive Brokers’ blended rate near 5%, the box saves a few hundred dollars on this bridge, and the choice comes down to which instrument your broker supports cleanly.

The table leaves out one difference that has no price. Borrowing keeps the shares invested, so a market drop during the bridge still hits you, and the portfolio is also the collateral. At bridge size that is rarely dangerous: with $100,000 borrowed against a $1,000,000 account and a 30% maintenance requirement, the account could fall about 86% before a call. Broker house rules can be raised without notice, so treat that as the optimistic bound.

The long-horizon version: a box instead of the mortgage

The pitch for box spreads as home financing compares a five-year box rate with a 30-year mortgage rate and stops there. On August 19, 2026 Cboe’s December 2031 box index read 4.64%, against 6.67% for a 30-year fixed in Freddie Mac’s survey the week before.¹⁶ The tax comparison makes the gap look wider for jumbo borrowers. Mortgage interest is deductible only on the first $750,000 of acquisition debt, a limit Congress made permanent in 2025.³⁷ On a $1.3 million loan, the top $550,000 carries no deduction, so moving that slice to a box gives up nothing on the mortgage side.

For one modeled buyer moving $300,000 of a jumbo loan to a box, with $1,000,000 of other acquisition debt, no realized gains to absorb the box loss, and the same rates, the box costs about 4.29% after tax against 6.67% for the mortgage slice. That is roughly $7,100 a year. If the replacement box after year five cost more than about 9.4%, the mortgage would have been cheaper over ten years.

The rate gap is real. The trade still fails for most households, for reasons the rate does not show:

• The whole balance is due in year five. SPX listings run to roughly five years, and 79% of box volume expires within six months.¹⁰ Repaying a bullet from anything but existing assets means selling a new box at whatever rates, collateral values, and broker rules hold then.
• The refinancing risk peaks in a crisis. Box rates sit above Treasuries by a spread that widens when dealer balance sheets are constrained. Cboe reported it at a multi-year high of 69 basis points over three-month SOFR in August 2026.¹⁰ A stressed market is when your collateral is down and the roll is most expensive.
• The collateral moves and the house does not care. A mortgage amortizes, cannot be called while you pay it, and is indifferent to the stock market. The box is secured by a portfolio marked to market daily. With $300,000 borrowed against $1,000,000, the modeled account could absorb a 57% decline before a call, against 86% for the bridge example, and it carries that exposure for a decade instead of a quarter.
• Cboe’s own staff advise against it. Its director of derivatives sales wrote that short boxes “should be considered for short duration and clearly defined needs, not for long-term financing or lifestyle leverage.”⁸

What your mortgage lender will see

Borrowing against a brokerage account for the down payment is allowed. Fannie Mae’s Selling Guide states that “borrowed funds secured by an asset are an acceptable source of funds for the down payment, closing costs, and reserves,” and that when the asset is a financial asset, “monthly payments for the loan do not have to be considered as long-term debt.”⁹ Two conditions follow. If the same account also counts toward your reserves, the lender reduces it by the amount borrowed and related fees. And the lender must document the loan’s terms and that the funds reached you.

A box raises a practical question here, because nothing on a brokerage statement says “loan”: it shows four option positions and some cash. Tell the lender before you open one, ask how it wants the obligation documented, and ask whether a jumbo investor adds rules of its own. Opening the box after closing to refill your cash until the vest arrives avoids the underwriting question entirely, if the lender’s reserve test can be met without that cash.

When borrowing for the down payment is a mistake

• You cannot name the repayment. A vest on the calendar, a bonus in the offer letter, or a signed sale on a previous home qualifies. “The portfolio will be bigger by then” does not.
• The repayment depends on the job. Unvested equity stops if employment stops. Keep a fallback that does not: lots you are willing to sell, at a tax cost you have already priced.
• The loan is what makes the house affordable. If the purchase only works with a permanent loan against the portfolio, the house is too expensive for the plan.
• The bridge is small. Below roughly $25,000 the bid/ask on an off-the-run box and the risk of a construction error outweigh the saving over an ordinary margin loan.
• The shares are near basis. Then selling is nearly free and simpler than any loan.

How Summitward helps

A bridge loan is a liability with a date on it, secured by the same assets a plan counts on. Seeing it next to the purchase makes the repayment date and the fallback concrete.

Frequently asked questions

Is margin interest deductible if I use it for a down payment?

No. Under the tracing rules the interest follows the use of the money, and a home purchase is a personal use. It is not mortgage interest either, because the debt is not secured by the home.

Can a box spread loan count toward my down payment?

Fannie Mae allows borrowed funds secured by a financial asset for the down payment and does not count the payments as long-term debt. Your lender still has to document the obligation and will reduce your counted reserves by the amount borrowed. Ask before you open the position, since a box does not look like a loan on a statement.

Why not sell the shares and buy them back with the vest?

That works, and the higher basis means part of the tax is only paid early. The real cost is the deferral you give up, which is small for lots you would have sold soon and the whole tax for lots you would have held for life. The calculator prices both.

Does the $3,000 capital loss limit matter for a short bridge?

Usually not. A three-month box on $100,000 costs about $1,000, inside the limit. It matters for multi-year boxes, where the annual cost exceeds $3,000 and the rest carries forward unless you realize gains.

Is a five-year box cheaper than a mortgage?

On rate, in August 2026, yes by about two points. It is a bullet secured by a portfolio rather than an amortizing loan secured by the house, and the rate after year five is unknown. The second tab of the calculator shows the replacement rate at which the box stops winning.

Key takeaways

• Name the repayment first. Borrowing for a down payment makes sense only for a gap that closes on a date you can point to.
• The deduction follows the money. Margin and pledged-asset interest spent on a house gets no deduction; a box loss is a capital loss whatever it paid for.
• Selling costs the deferral. Rebuying with the vest resets the basis, so the cost of selling depends on how long you would have held the lots.
• For one modeled $100,000, three-month bridge, a box cost about $700 after tax and selling cost $7,400 to $14,300. Your inputs will move both figures.
• A box as part of the mortgage trades a lower rate for a bullet maturity. The $750,000 cap widens the after-tax gap for jumbo borrowers without changing the refinancing risk.

Related guides

• Box Spread Loans: What They Save Depends on the Rate You’re Escaping covers how a box works, the rate conventions, and the execution checklist.
• Funding the Down Payment: Gift, Family Loan, or Sell the Stock? compares the sources that do not involve borrowing against your own portfolio.
• Personal Leverage: Margin, Leveraged ETFs, Lifecycle Theory, and Forced Selling covers what a margin call does to a portfolio that has to sell into a decline.

Sources and method

1. Cboe, Box Rate indices, one index per listed SPX expiration. Values as of August 19, 2026: December 2026 4.24%, December 2031 4.64%. Indications of where the market has priced, not quotes you can lift. Cboe
2. Treasury Regulation 1.163-8T, allocation of interest expense among expenditures. Debt is allocated by tracing disbursements of the proceeds to specific expenditures. GovInfo (PDF)
3. Internal Revenue Service, Publication 936, Home Mortgage Interest Deduction (2025 returns). Secured debt requires an instrument that makes the home security for the debt and is recorded or perfected; acquisition debt incurred after December 15, 2017 is limited to $750,000 ($375,000 married filing separately), across main and second homes. IRS
4. Internal Revenue Service, Publication 550, Investment Income and Expenses. Nonequity options, including broad-based stock index options such as the S&P 500, are section 1256 contracts: marked to market at year end, 60% long-term and 40% short-term. IRS (PDF)
5. 26 U.S.C. §1211(b), limitation on capital losses for individuals: net capital losses offset ordinary income up to $3,000 a year ($1,500 married filing separately). Cornell LII
6. Freddie Mac, Primary Mortgage Market Survey: 30-year fixed 6.67% for the week ending August 13, 2026. Freddie Mac
7. Center for Agricultural Law and Taxation, Iowa State University, One Big Beautiful Bill Act Implements Significant Tax Package. The Act, signed July 4, 2025, makes permanent the $750,000 acquisition-debt limit on the home mortgage interest deduction. CALT
8. Rick Rosenthal, Cboe, SPX Box Spreads: What Every Advisor Should Know, March 12, 2026. Cboe Insights
9. Fannie Mae, Selling Guide B3-4.3-15, Borrowed Funds Secured by an Asset. Acceptable for down payment, closing costs, and reserves; payments need not be counted as long-term debt when secured by financial assets; reserves reduced by the proceeds and fees when the same asset is counted. Fannie Mae
10. Cboe, SPX Box Spreads market statistics (data as of June 23, 2025: 79% of box contracts expire within six months), and Mandy Xu, Week of 8/3/2026: The Fed Holds but Bond Volatility Breaks Higher (box yield premium of 69bp over three-month SOFR, a multi-year high). Cboe statistics, Cboe Insights
11. Method. The calculator’s arithmetic lives in web/src/lib/down-payment-bridge-math.ts and is unit tested, including every figure quoted above. Bridge interest is amount × rate × months / 12. The box loss is valued at the capital gains rate when realized gains absorb it, otherwise at the ordinary rate on up to $3,000 a year, with any carryforward given no value. The cost of selling is the tax now less its present value at the later sale the lots would otherwise have had; lots held for life forfeit the whole tax. The long-horizon comparison uses after-tax rates on the same balance, ignores the mortgage slice amortizing, and holds the deductible share at its starting value. Pledged-asset and margin rates are the August 2026 schedules tabulated in our box spread guide. Nothing here models commissions, bid/ask, or broker-specific margin on a short box, and nothing here is tax, legal, or investment advice.