Incorrect installment

Dear MTN

I recently cancelled by contract since it was expiring and to my surprise the installed increased and when I called to ask I was told by a consultant that the was an added item on my contract which it cannot be explained and when I asked how it got added I didn't receive any answer. No once assisted me and yet they debited my account with an incorrect debit installment and when I was to log a compliant the consultant told me she cannot assist me with that. I need a rebate for the amount which was unauthorized on my contract