avis robing me

I rented a car for 3 days. On a rival the place was a mad house they couldn't keep up so I got my car and no one showed me to the car or inspected the car with me. On return a dent on the side was pointed out to me so I wrote on the form that it was there and that no one was with me to inspect the car on pickup. Now the sent me sms that I have to pay X amount for the damage so I phoned them and questioned it so they said they would get back to me. Today they just deducted a larger amount then what was asked in the first place. I haven't tried phoning but no answer this is bull.