Negligence at MTN store

Last year november in a mtn store in Davenport I was assisted my a mtn clerk called Smanga and we set up an upgrade for my contract which was previously with nashu mobile. I selected the mini S4 and told him for every month I want my line to be open but I want to be softlocked once I reach R1200.00 so that I do not owe anything more than that for data and call usage combined, just as my line was set up at nashua mobile. He then told me he will set it up. From december to January I recieved SMSs alerting me I had been softlocked for reaching my limit of R1200.00 but on febuary 3rd they took R2800 from my account and further claimed that I owe another R2800. My question is how could they bill me for so much when their store clerk Smanga had set up a usage limit which the sent me an sms saying Ive reached for each month. Either Smanga did not do his set up correctly or something is mixed up. Please help before I wrongfully become black listed.