Thats a great question, I don't know the answer, the obvious 2 possibilities are (a) Lender keeps it, or (b) You keep it.

I suppose it all boils down to, what is the Lenders true motivation? If he is really just focused on being a traditional fiat lender (with BTC collateral option), then perhaps the borrower keeps the overage. (ie. Once the loan is satisfied the borrower keeps all assets he purchased with the loan).

However, if the Lenders true motivation is to stack sats, then he would keep it. (In this case, the theory would be the Lender "had claim" to the Bitcoin since it was a collateral asset and he winds up taking control in exchange for property release).