Money-making by changing invoice date

MTN changing invoice date to 5th of the month, has caused my monthly bill to increase by R118!!?? I am on contract and have always paid R365.80 per month. Now, I have been debited with R483.80!!?? Just because they changed the invoice date!?? They state on web-site that the debit order amounts will NOT CHANGE with their switch of invoice date!?? Pure money-making rackert. Dont use MTN for contracts; go pay-as-you go