Car sold, now the buyer wants money back!

Hello, I sold a car today. It was the accidented car of my grandparents (my grandpa had mistaken brake and gas; therefore the car was totally broken at the front. Hood was slightly broken, but the bumper was totally crooked and the fender was too!). I sold the car for my grandparents so they would not be pulled over the table – and now one tries to me. So the buyer came today, looked at the car, bent the car a little bit on the spot (!) and fu The car wasn’t traffic safe in my eyes anyway, because the headlight and turn signal were broken, the bumper hung down ect. Well, I don’t care. He gave me the agreed money and drove off. Now he called and said the car had a damage at the back right. There was a slight impact on the side wall. Maybe my grandpa got there somewhere. But I don’t know either and if you see it then you can only look at it closely. but money back because he didn’t see this “damage” on site! We sold the car like this and he looked at it on site, but didn’t see anything?! We did a Stadard sales contract with the note “Accident damage in front unrepaired” and “not traffic safe”. How should I behave towards the buyer? The car was almost 14 years old and certainly still has a Macke, Delle, scratch ect. MFG