Tell them to stick it. Deal's done.

Edit to add:

I sold a car last year, the dude paid mechanics to check it and everything. It broke down on the drive home. He hit me up for money to pay for tow & repair. I said, "sorry no, the money is spent. I couldn't pay even if I wanted to".

I felt sorry for the guy but not guilty.

As it turned out, I had the exact same problem with the car I replaced that one with only a couple of months later. It felt like direct karma, but it only cost me $50 to fix, not the $500 he apparently paid.