Overcharged & Inncorrectly billed

Incorrectly billed. The sales team at MTN in waterfall like to sell you a contract at a certain price(Price Stated on signed contract). Then you get a sms(no invoice or statement)that the first month charge is almost 4 times the amount. They will take the money off your account he said. MTN will only refund/credit your account in 21 working days, he said. Your contract has not been billed at the special rate(the package I chose was on special)THEY he said have billed you the full amount without the discounts of the package the amount is R739.00 monthly for the contract but I have to pay R3900 and in 21days they will refund me the monies, I said I don't have that money in my account its going to bounce, well he said there is nothing more they can do.. I'm shocked and upset about the heartless manner I was treated like why get upset for such a little money, whether is be R20 or R3900 its called *****ing and breach of contract/********** I wanted to cancel my contract for the inconvenience, but I was informed that it would cost me more if I did that by the sales guy at MTN Waterfall Shopping centre When I requested to see the manger about my concerns I was told to contact the customer care department and inform them of my grievances. what if you don't have the money to pay? why cant they pass a credit immediately knowing there is a problem, which is them at fault. why should I the customer have to wait 21 working days to have the billing rectified and have account credited with money I never had in the first place?