I sold a used van to a private buyer, "as is". He signed a bill of sale stating as is", I signed over the title and received payment. An hour after he left, he called me to tell me the van had thrown a rod on the highway. I did reveal, before the sale, that the van may have a bad cylinder. He want's his money back. I say no, since I would be getting back a worthless van. I agreed to refund $500 toward his cost of gettting the engine replaced. Am I obligated to retrurn the money, or is he out of luck?