Same thing happened to my friend. Budget sent him a bill for $1500 worth of damages to a car he had returned 4 months earlier.
He called them and told them that he wasn't going to pay for something they had no proff he had done. After all, the attendant did do a walk-around of the car and didn't notice anything, so how does he know it wasn't someone else ho had done it? After the phone call, Budget told him to forget about it and dropped the matter.