Does the spread thin out faster than expected when the underlying has a higher balance than you thought

The assumption doing the most work here is that the spread compensates you for credit risk on the full $265,000, when the structure means your actual exposure to buyer default is asymmetric across two different loan pools with different paydown speeds.

What you have identified is correct. A 30-year loan originated in 2021 at a low rate is front-loaded with interest like any amortizing loan, so principal erosion in years one through six is slow. Your $15,000 gap at month 48 is not a rounding error, it is a position where, if your buyer defaults at year four, you owe $205,000 to the underlying lender but your buyer has paid down your wrap note by far less than that on a $265,000 balance at 7.5 percent. The buyer's equity cushion at that point depends almost entirely on appreciation, because amortization is not building it fast enough to close the gap.

The risk you did not name directly: if the buyer defaults and you have to foreclose, your cure-and-carry cost during a contested foreclosure eats into the spread you collected. In California, the foreclosure timeline can stretch depending on process and any litigation, so you need to confirm current timelines with a California real estate attorney. That carry cost, principal plus interest on the underlying, is real cash out of pocket against a buyer who is already in default.

The structure is also subject to California's seller-financing disclosure requirements and wrap-specific legal constraints. A California real estate attorney needs to construct the documents, not review them after the fact.

On the mechanics: have you modeled what the spread actually yields net of the underlying payment across the full hold, discounted, rather than looking at gross monthly spread? The spread on the $218,000 at 340 basis points sounds clean until you price in the asymmetric paydown and the default scenario at year four.

What down payment is the buyer bringing, and does it cover the realistic gap between the two balances at a plausible default point?

Yes, this is a real effect and you are reading it correctly.

Early in a 30-year amortization, nearly all of each payment is interest. Principal reduction is slow. On a $218,000 balance at 4.1 percent, the monthly principal portion in year one is roughly $380. By month 48 you have paid down something closer to $13,000 of principal, not $28,000, which is why your model and reality are $15,000 apart. The underlying balance just does not fall the way a later-stage loan would.

What that means for your deal: the spread you are earning is real, but your net equity cushion, meaning the gap between what your buyer owes you and what you owe the underlying lender, compresses more slowly than you may have priced into the deal. If your buyer defaults in year four and you need to sell or foreclose, you are exposed to that $15,000 gap plus transaction costs. If the buyer's down payment is thin, that gap is not covered.

The piece worth confirming: what is the due-on-sale clause situation on the underlying loan? The strategy guide flags that the underlying lender could call the note, and on a four-year-old loan the servicing record is visible. A real estate attorney familiar with wraps in California should review how the existing deed of trust reads on transfer and what your exposure looks like if the lender accelerates. That is a licensed-professional question, not one to model away.

The Rena strategy guide for wraparound mortgages covers the payoff-exposure and due-on-sale risk in more detail under the considerations section, worth a re-read given where you are.

What is the buyer's down payment as a percentage of the $265,000 sale price? That number changes how much of this gap is already absorbed before month one.

What is your buyer's down payment on the $265,000?

What happens if your buyer stops paying in month 30 and you still owe $207,000 to her lender?

The $15,000 discrepancy you found is real but it is not the exposure itself, it is the signal that your original amortization input was off, and everything downstream of that input compounds the error. On a 30-year note from 2021, you are in the steepest interest-heavy stretch of the schedule, which means months 37 through 60 on the underlying are paying down maybe $300 to $400 in principal per month depending on the original note amount. So by month 48 you are not just $15,000 short on paydown, you are also $15,000 further from the crossover point where your buyer's equity cushion starts to matter as a real buffer. The wrap works as long as your buyer keeps paying, but the unwind scenario is what you are actually pricing when you set that 7.5 percent rate, and the unwind scenario now costs more than you modeled because the gap between what you owe her and what your buyer owes you is wider for longer.

The number I would want from you is the buyer's purchase price relative to current market value in Fontana right now, not the $265,000 contract price but what a distressed sale of that property would net you in 90 days if you had to move it. If that number is comfortably above $218,000 plus your costs, the slow paydown is an annoyance and a cash flow timing problem, not an existential one. If that number is close to $218,000, the slow paydown means you have almost no margin of safety at the exact moment in years three through five when a buyer default is most statistically likely, because that is when life events hit and equity is still thin. What does the fast-sale number look like for that area right now?

The misconception worth correcting is that the spread "thins" when really the exposure grows, which is a duration problem rather than a yield problem.

Your $15,000 gap at month 48 is not eating your spread, it is lengthening the window where a default forces you to cover a payoff you cannot fully recover from the buyer's equity position.

The spread does not thin, it stays fixed in rate terms, but your equity cushion erodes relative to where you thought you would be if the deal unwinds mid-term.

At month 48 with the underlying at $205,000 instead of $190,000, your recovery position depends almost entirely on what the buyer paid in and what Fontana values do between now and then, so the $15,000 gap matters a lot if values are flat and almost nothing if they ran up 10 percent.

What is the buyer's down payment as a dollar figure, and did you structure a balloon or is this running full term?

Ran almost this exact scenario on a wrap in Mesquite back in 2022, underlying was a 2020 origination at 3.25, and the slow paydown on those early years burned me the same way on paper. What saved me was that my buyer put 11 percent down, so the gap between what I owed and what I could recover on a default was actually shrinking from the buyer's equity side even while the underlying crawled.

What's your buyer bringing to the table at close?