If he purchased the car through a bank with a loan. It is his. He will not be able to stop payment on that check. You can not stop payment on a check issued by a lender after you've received the property and signed the loan. He and the bank would both be liable to get sued by the seller. You have signed and sealed that it is AS IS. Then he can not return the car for petty shit.

About 2 years ago I sold my Mazda 6 for 15k. The kid purchased it from me with a loan from his credit union. It was a week until the loan balance turned to $0.00. Hell.. I refinanced my Nissan Cube to my current bank and it was around 2 weeks for the old lender to clear my check from the bank. I think when you get a payoff amount they give you the 10-day payoff.. which will give them a few days to mess around with the check.

If all else fails... Sue him for damages to your car.. then sell it again.