Mtn system error, but my lines get suspended

i upgraded my contract on 10/03/15 at MTN Cavendish, by Valencia Ernstzen and requested a R1 usage limit to be set, she confirmed it had been set. but on 19/09/15 Mtn suspended my lines, i went back into the store, they said my bill was over R4000, Valencia confirmed she had set the limit in march, she said it was their system error..she gave me proof of the usage limit set, she requested a credit for all the xtra charges, in november her manager jainap called and said its not their fault, mtn will not pass the credit and i must pay the bill, and it is now with the legal office. This is unacceptable, why must i be held responsible for their system error? I have continued paying my normal contract fees and havent skipped a month. They breached the own contract by suspending my lines without notification, and now handing it to legal department as well. There is also a credit limit on my account of R2900. how can the bill run to over R4000?? another system error. I have tried sorting it out via call centre, and another branch, but to no avail. Please help me.....This is having a negative effect on my Credit rating, and its not even my fault