First of all, I am not going to pretend to know the halacha, but a goy is obviously not bound by halacha. In Florida, and I suspect NY as well, a contract that says Person A will pay person B “fair market value” is not enforceable, unless it permits someone (besides a judge, for example an arbitrator) to set that value if the parties do not agree because price is a material term that cannot be left to a judge/jury to fill in. So even if the contract says that, you cannot enforce it. Perhaps the Goy can because maybe halacha recognizes such a deal, and the Jew is bound by halacha, but at least in Florida, it cannot be enforced.
So the practical answer to Joseph’s question (at least in Florida) is that even if the contract said that, and a Jew had to comply with Halacha and honor such an election by the Goy, it is unenforceable in a Fl court.
I do not know how rabbis structure these deals, but the common sense way would be similar to what is referenced above (ie a mortgage). You sell the Goy your chametz for 100,000 but he only makes monthly payments, and the first payment is due in 10 days (or 8). Upon failure to make the payment after pesach, there is a default, and the property goes back immediately to the Jew. The end. In that scenario, the jew cannot insist the Goy pay anything, as the contract provides that the sole remedy on default is a return of the property (collateral). On the other hand, the Goy has the right to pay and keep the property.
There are many real loans that are handled in this way, so to say its a mockery because the jews only remedy is the collateral is not accurate.