If i sold my car to someone that have signed the pink slip title? can he ask for his money back after that?

he signed the pink slip title only? is that considered as an AS IS sale? do he have the right to ask for his money back? Ohh, one more thing, the DMV has asked the guy ( the buyer ) to do a new smog check, so the guy did it, but it failed. And now he is asking me for his money back, what should i do?? P.S: I live in California.