Charged incorrectly

I picked up a car at the Cape Town AirPort recently. The service was very friendly and efficient. I signed the contract and was then sent outside to get the car. I had to wait awhile for someone to come and check the car, so I could leave. Eventually someone came and walked with me around the car, very quickly and said all was fine and I could go. He didn’t ask me to sign anything nor did he have any document, like I’m used to. I was in a hurry and didn’t question this, stupidly in hind sight. When I dropped off the car at the Herm**** depot four days later, the Avis person and I walked around the car. He saw a mark or scratch on the front fender. I told him it had definitely not been me and told him the story. I also wrote this on the contract and left. I phoned Avis customer care, but they referred me to the Airport branch. I tried several times to get through, to no avail. I also never heard from them, until I noticed on my bank statement they had charged me around R2000.00 extra. This with no explanation. I still feel, as I did not sign any document stating that the car was fine, that it’s not right to charge me for something that was not my fault. And Avis cannot prove that it was I.