Avis holding me liable for the damage on the car that i did not do.

I have hired a car at Avis OR Tambo Airport on the 24 december 2017. i picked up the car around 16:00 and the lady who was helping me just captured my credit card information and requested me to capture my email address, i did not sign anything on that day and she gave me the car keys and said i must go and collect the car at the bay where is packed, no one went with me to evaluate/assess the car. so when o got home i realised that the car had a dent on the left mirror and bottom of the body and on that time i told myself that because the dent is bigger obviously they know about it and is recorded somewhere. The problem started when i returned the car on the 27 December 2017 at Johannesburg downtown , they said i am the one who scratched their car and they could not find anything from the previous drivers, meaning nothing was noted about the damage, if they did is gonna take time to reflect on their system, thats what they told me. my problem is Avis deducted me money for the damage and i returned the car with full petrol tank but still they deducted me money for the damage that i did not do plus they took my deposit money also. Total amount they took without my concern is R6908.00 while they told me they wont deduct any amount without concluding their investigation. The rental agreement amount is R1229.26 so i dont know why they took that R6908.00.