DATA BILLED ON ACCOUNT

Good day<br> <br> I was always at Nashua Mobile - they closed November and transferred me to MTN branch.<br> <br> I had a arrangement with them from the beginning of my contract that I will load my own data and it mustn't be billed on my account. It was never a problem.<br> <br> Now I received my MTN statement and the total is dubble as it usually was. They billed the data on my account ( R417 ) and I still loaded my own data. I cant pay that amount now they said they will divide that balance in 3 months but it is still to high for me to pay and it doest make sense. R925.78<br> <br> My total every month is about R430. Now it is R950. And for the 3 months coming im going to pay R750 as they divide the amount for me.<br> <br> And from the beginning it wasn't my fault that they got there work wrong. What can I do. PLEASE HELP.