Surplus charge for fuel unwilling to refund

Absolutely shocking after service. They billed me for "topping up the car". I had filled the tank up before handing it over. They state they needed to "top up" the tank with over 13 litres of fuel for over R300. This is over fueling and they did not state I need to over fuel. The contract does not define that one needs to full beyond the first click at the pump.