Charged for fuel

I hired a car from Port Elizabeth Airport. On the contract it said I needed to fill the car with petrol and keep slip. On the way to the airport I filled up the car with fuel at a Fuel Station in Heugh Road. When delivering the car the lady who served me wouldnt accept my slip as she said I had to use the fuel station at the airport. At no time was I told that was the case. Upon receiving my final invoice I was billed R116 for fuel. There is no way that this is the amount of fuel needed as the indicator when I delivered the car was full. I sent an email to Hertz and received no response.