In my poor post-grad days this happened to my wife. She backed into another car and we appealed to him to let us cover the expense and not get our insurance involved (second accident for her and L.A. auto rates-'nuf said). He agreed, but wanted cash. We naively paid after he produced an estimate from a body shop.

He then reported it to insurance anyway. Long story short, he got paid twice and we got the shaft. Live and learn. No cash transactions with strangers...ever.