MTN contract billing still a issue

I upgraded my contract over the phone 3 months ago, the first month they deducted double the amount from my account. I called in to retentions department who did my upgrade and they made a mistake on loading my package on the system. this was apparently fixed, I will be credited on my account for this month. guess what they deducted the same amount again even after I called in last week to make sure they will deduct the correct amount after the credit. this is unacceptable. I have not budgeted for this, I am falling short again this month. what going on here MTN. I want hard documented proof that this is correct or I will cancel my contract.