Rental deposit not returned

Rented a car at lanseria airport. Was advised by staff to hire and pay for extra 2 days to get additional mileage and done so. Dropped off vehicle with about 250kms over the three days total mileage we paid for and was billed for all the additional kms and they ignore the fact that we paid for 2 additional days. They keeping my R8700 deposit and also asking me to pay an additional R2000 but not wanting to refund the 2 additional days I paid for even though I used the vehicle for one day. They email saying that head office decided that the decision stands and they keeping my deposit after an investigation but they did not contact the branch at Lanseria airport to find out what they advised us? I spoke to the manger Zoleka at Lanseria airport and she told me she hasn't looked into yet and will do so and get back to me and haven't responded to or chatted to headoffice about the matter so how did they investigate? There are many complaints here with the same issue - them not wanting to pay deposits back so it seems like normal procedure for them dam *******!!!!!