Wrongfully billed

On the 01/08/2015 I upgraded one my contracts with MTN. I bought a Samsung galaxy A3 for my mom and I asked the agent for a top up contract which in return he rep**** saying he will put limiters at 0.00 for the contract , I did this knowingly that my mother is not technological advance but last week <br> Friday 18/09/2015 I recieved an unbilled data usage of about R1900 I immediately checked with MTN 808 who confirmed this. the following day I visited the MTN cresta where I had made this upgrade to check excetly what is happening and the told me that my line was capped meaning it has a limit of 0.00 and I told them about the data bill I just received and they were also shocked because from their system they could see the limit and the date at which it was set together with the ref *** that clearly states that limiters were indeed set on my line but now MTN wants to hold me liable for this usage but on what grounds because this not my fault and that as a customer I asked them for a top up package , now I am seating with R1900 bill that I do not know how am I goner pay , I have never had issues with MTN but after this left me very dissapointed and as result I am very stressed now