Poor service and extra charge for scratch not caused by ourselves.

We recently rented a car from Avis at King Shaka Airport Durban. We arrived around 20h30 (night), car was pre booked. We arrived at Avis and was given a different car from what we requested, but that was okay. It was at night and nobody was around to escort us to the vehicle and do an inspection. The car had numerous little dents and scratches as per the report we were given, one of which was on the front left bumper. We used the car for a few days and had no incidents what so ever. Upon returning the car the guy inspected the vehicle and brought to our attention a little scratch on the front left bumper (less than a cm in length) and advised that this was not there before. Remember there were many other scratches on this vehicle. I said to him that we didn’t do anything wrong and that must have been there from before. A few days later we received a report from Avis for R2100 for that scratch, which one would even say is negligible (and we didn’t cause that scratch). I looked at the report provided prior to us taking the car and it clearly reflects a scratch in that area of the car. We contested the initial report from Avis but got no response whatsoever. Now we get an invoice saying we owe R2100 for the scratch. We obviously have no recourse. I suspect they will use this money to repair all the scratches on the vehicle. I’m so disappointed and helpless, what do I do?