MTN failed me

I upgraded and did a trade in for a Samsung product. The phone was supposed to be R450 - R650 at most after trade in, however MTN acknowledges the fact that they messed up and they are charging me R1000+ for the device, which they should not be. Its been going on for the last 4 months now, and I have to pay the bill so it does not affect my credit record. I have over paid now by R2000+ with the next bill, and MTN is just giving me the run around. I need to go back to the store to have them sort it out. I go then I am told a ticket was NOT LOGGED, then its logged I need to mail this person, I mail them and they ignore me. I call the call center and I am told nothing was logged, and I need to send the documents in. I have no documents because this was supposed to be sent to me, but this was not done. There were like 5 contacts created and cancelled on the day. Can someone please assist me. I have been with you guys 20 years + with multiple contracts, and I never missed payment or gave you guys grief.