I am assuming he bought the car after a test drive, in which it performed well and he was happy enough to buy the car? Then IMHO you can't be held responsible for anything that happens after you have sold it, having done business with you before, albeit for small items, I know you are not the sort of guy to rip people off......

A deal is a deal, he should stick to his word and give you the £50, bit low if he don't IMHO...