MTN CONTRACT CANCELLATION

Good day. In 2003 i took a home wifi contract with MTN and it was not connecting most of the time from where i reside so i decided to cancel the contract. I called in for the cancellation and they communicated that i should pay the cancellation fee, of which i did pay, only to find out that i was made to pay more than i was supposed to because the last statement that was sent to me for that payment had a negative figure and i have visited all MTN shops at 3 different malls to try and resolve the issue for me to get that money back but they just go quiet after taking my details. Surprisingly on all the MTN shops i have visited, they do confirm that MTN is owing me the negative balance from overcharging me during the cancellation.