Dispute

I rented a car from AVIS- Cape Town International Airport. The agent who was assisting me told me that the car have a dent and she showed me on their system where she had noted that down and handed me the keys. She told me that I was going to receive a copy of the agreement via email of which I did and I din't go through it because I wasn't suspecting wrong, she didn't give me any hard copies. When I returned the car I was told that I have damaged the car and I told them this was an existing damage. I was asked to write a statement of which I did. Few days later I was charged R6800.00 for the damages. I explained again to them the car had an existing dent that was noted down on their system hence I agreed to take a car. I was asked the name of the agent who assisted me and I told them that's not fair because she is their employees they should know who assisted me on that day. I was then told they need to do an investigation, few days later I was told the charges will remain since there is no documentation on that says the car had an existing damage and they asked me to provide it but how am I going to provide it since i was not given anything in writing. Unfortunately I didn't suspect anything but now I see that this was a plan to rob me. Until now they are refusing to refund me.