Being Charged exorbitant amounts due to errors made by MTN

It's been a month and 2 weeks now trying to solve a problem that was caused by MTN but everytime I'm giving false hope that never receives any remedial action. I took out an upgrade contract in December 2017 and I was supposed to pay R500.00 but I'm not being debited R699.00 because apparently the agent who helped me take out the contract made a mistake and this mistake is forever being amended, till this day I'm not getting joy for my money.