unauthorized deduction

I rented a vehicle at Dollar thrifty in Centurion on the 22 December 2017 to the 02 January 2018. I returned the vehicle on the 2nd of January as per contract. When I returned the vehicle it was empty. I was expecting that the vehicle I rented will be filled using the deposit that was debited from my account on the day of collection. To my surprise dollar thrifty deducted the petrol money from my account making my balance to be negative because I keep my money in my savings pocket. I went on to call Centurion branch where I took the vehicle to find out what was going on. I told them that money has been debited and deposit not returned. They told me that did not happen. I told them that my account is negative as we speak. They were rude to me telling me that I am not civilised . I am frustrated because no one is helping me. From what I know petrol is taken from the deposit.