Incorrect Fuel Charge

I hired a car from Thifty at the airport. I returned the car 9 days later and filled the car (for the 3rd and last time) at the Sasol at the airport - approx 20 metres from the return bay. They then charged me R211.00 for the refuelling of the car and expect me to provide proof of refuelling whilst I had the car. It is totally impossible for the car to have taken any fuel, and the final slip (and one before) have irregular amounts for refuelling (as opposed to a specific round figure eg R200 if I was not filling up). I have queried and after a week, they send me an invoice reflecting the refuel charge (not proof of refuelling). I have sent them two of the slips, but not the one when I paid cash. Surely they need to prove that they refuelled -- I did 1038 kms over 9 days so they cannot use my mileage to work out the refuelling. I feel I am being **********.