Damage to rental car

I had a rented car from Avis after my car was in an accident. the day I returned the car to Avis, the person who inspected the car told me that there is a bump on the lower bottom of the front right bumper. I for sure knew I never bumped this car. But the insisted that I sign a form for the alleged damage. Later in the day I went back to the car to check out the damage just to make sure if I was in the wrong. the alleged damage was just a black mark which I removed with car polish. I then phoned Joanne the next morning and asked het to go check the car. She then told me that I am 99.9% correct that there is no damage to the paint of the car but they will still have to send it for a buff. Why must I pay for a car buff if the alleged damage was only at the bottom of the car? I was slashed with a bill of R750 for a full car buff, and I don't think it is fair. I refuse to be held liable for this bill because in my mind \Just because they can"and because they want to slash me. I refuse to pay because I actually took pictures of the alleged damage and they need to proof beyond reasonable doubt that this car was indeed damaged."""