I snap a couple of quick photos when I return rentals becs the standard rental contract allows the company to assess damages after the fact and, if you read the contract, you agree to pay them. The pictures will help in a dispute. You can also initiate a chargeback on your credit card if a charge does show up. However, if it's for anything significant, you will likely lose the chargeback dispute due to the very specific contract language unless Avis wants to acommodate you as a matter of customer service. Put simply, there are no great answers and the Avis employee who presumably dinged the car while cleaning/parking is not likely to admit error.