My contract closes in 21 days while the end buyer wants 60 of diligence

The extension is the cheapest fix and the double close is the expensive one, so work the extension first. Prepare an amendment with a new closing date and nothing else changed, get it signed before the current date passes, and have your closing agent hold it. Once the original date lapses without an amendment your equitable interest is arguable, and a seller who's had second thoughts will treat the contract as dead.

But the structure you actually want here is the builder taking assignment now with his own inspection contingency, not you carrying his diligence. Assign at $27,000 with $2,000 or $3,000 of the fee non-refundable on assignment and the balance at closing, and let him negotiate the perc timeline directly with the seller. That moves the health department risk to the party who cares about the answer and gets you paid partially regardless of the soil result. He'll push back. His pushback tells you whether he believes the parcel passes.

On the numbers, a $9,000 spread on an $18,000 buy invites exactly what flint said. If the deal is assignment-only and he sees your figure, either agree the fee openly up front or plan on a double close and price the second set of closing costs, transfer taxes where the state charges them, and any recording fees into your spread before you decide. Transactional funding at $18,500 for two days often costs a flat fee plus points that eat a fifth of your margin, and some funders won't touch land at all.

The risk you haven't named is the estate. An out of state heir who's been paying taxes for nine years may or may not have completed probate, and if he can't convey clean title no timeline helps you. Pull the title commitment now, before you spend anything on the extension conversation.