A very similar incident happened to me about 3 years ago.

I returned a car, the person walked around vehicle, gave me my receipt, and I left.

A month later, I get a letter and invoice for damages to vehicle.

I did the same thing as a poster above, disputing any damage incurred while the vehicle was in my possession and that the vehicle had been checked in by someone at Avis who had done an inspection at time of return.

It took a letter from me with some threateneing legalese language as they continued to pester me about it without showing ANY evidence, but they eventually dropped the ridiculous claim. it was a headache for sure.