Incorrect billing, impossible!!!

I've been a contract customer with Vodacom for many years, I check my account all the time with the app, Friday afternoon my account was sitting at R900, I received an Sms that my data is out so I recharges(received the data I bought), plus I received an Sms saying that I reached my cap of R900(which vodacom confirmed), Saturday morning I checked my account which was at R1740, the call center say that their billing is 100% correct all the time so it's my fault, it's impossible for me to almost double my account in less then 24 hours, the help lines basically say that's the way it is which is a load of ****. Luckily my contact is month to month, if vodacom doesn't fix this theft from their side I will be canceling my line