I need a refund

I hired a car at Avis Johannesburg CBD on 28 July and returned it on 31.I left the car with a minor scratch on the black rubber under the bumper & on the 1'st of August I received an SMS saying I was billed R4016.42 for the damage,when I call to enquire they told me they replaced the rubber and resprayed the bumper

To my surprise when I request for a report that I signed when I left the car,it was tampered with. They cancelled what I wrote(from left to right) I don't understand why did they resprayed the bumper if they replaced the rubber They don't have proof that they replaced the rubber and resprayed the bumper, they say they only take pictures for damage that is more than R5000

Most of the things don't make sense to me,please help me

My rental number is:E ********** 55