Incorrect pricing

I was at Hillshaven, Westonaria yesterday and filled Diesel they filled R300 and the station read R19.79 per litre and the slip they charged me was R21.35 I called them and they are giving me another slip today with a different amount. I'm a customer and I always check how many liters I get for my money and now they disputing and trying to be correct. Please be careful when filling and check your slip.