Overcharged and refusing to correct the bill

Yesterday I took a ride with a total amount of R124 and when I was paying with R200 the driver didn't have change, and unlike other times where they will take the money and credit my account with the change, he advised that I pay R100 and I will pay the extra R24 to my future ride request and we agreed. When I wanted to request another ride this morning the amount owed was no longer R24 but R54, I have been trying to get that corrected the whole day today with no success to a point that I have decided not to use their services until this matter is resolved......