No fair treatment

We rented a car for 17 days with 100 km to drive each day. As soon as we discovered we would exceed the total kms (1700) we would drive our holiday, we asked if we could change the km to unlimited. This was no option. Fair point as we should do our homework better and should know this in advance. So we decided to return the car earlier with 1628 km driven. We tried at the helpdesk to change the contract for unlimited kms, but this was not possible. But they said to us we would get some money back because we returned the car 7 days earlier. But thereafter they charged us for extra km's, because considering their logic we did 628 km to much. This is a horrible service as they charged us for over 3000 rand. If we parked the car nearby and returned the car 7 days later, this could save us these extra costs, which is ridiculous.