I believe that a Rav I used to use to sell my chametz structured it as a down payment, with a balloon payment due after Pesach ended. If the balloon payment wasn’t made, the chametz would revert back to the original owners – not retroactively, as that would be a problem.
Since this was the contract, the Rav isn’t buying it back for you, it’s going back because the contract stipulates that. Since you authorized the original contract, you can’t change it.
A bigger question might be if you found chametz over Pesach. Let’s say you sold all chametz in your home to a non-Jew. Then, you find something that you missed while cleaning – let’s say a cookie was found inside a cabinet. How can you burn it? It doesn’t belong to you, it belongs to the non-Jew that you sold it to!