Invoiced incorrectly

I called mtn 05/05/2023 to cancel my contract and was advised to rather upgrade which I did. 15/05/2023 I received a call from mtn saying there was an error or mistake when they did the upgrade on 05/05/2023 and they had to reverse the upgrade and start over. End of the month I received an invoice and I was charged for the reverse and service for fixing the mistake they made when making the upgrade. Been calling MTN for weeks now and they also agreed that I was overcharged but no one wants to take accountability for the refund. I have been moved from 3 departments Customer service, Termination and Billing department and they all say they are not responsible to do the refund even though they agree that I was incorrectly charged.