Double billed YET AGAIN!!!

I recently did an upgrade on my account and I do understand I needed to make provision for pro-rata billing. However, when checking my account this morning I noticed my bill is R2950!!! I logged a complaint with the call centre and got a reference number ***, after a while, I received a notification saying the query was closed and I should contact MTN. According to the agent, My account was sent off for credit, but this is a "mass credit". I cannot afford for R2950 to be deducted from my account, which I explained to both the call centre agent and the agent who did the upgrade, both advised me to reverse the debit order. My question is, how do I reverse a debit order on MY account and let MY credit record look terrible because of MTN who billed me incorrectly. I need the bill adjusted and the money refunded to me, better yet to not go off at all and only the correct amount to go off. MTN sucks when it comes to getting back to their customers, yet call me all the time to take out another contract. This is not the first time MTN has billed me almost double.