DATA from 210 INSTALLEMENT TO 500

On the 28.11.2015 when I bought 1G of data using self service from my phone which cost about R120. I did'nt receive it as per request, 2mnths later i received a call from mtn advising me that I owe them about R1000. I explained the incident that took place last year & they insisted that i bought data of that amount. I don't undestand how is it possible that i purchased data of R1000 something when i only had R120 of airtime. They advised they will investigate and credit back the amount so that my it cancels all my arreas. In jan they emailed me & advised me that the issue is sorted. Now in feb they told me i was owing R460 which I then payed for arguments sake,but was suprised when the told me that for March my payment will be R560 as yet again I bought another data. So now my instalment is R560 iso R210, I feel is unfair as I signed a contract for R210 Top up which means I can't buy either data/airtime unless I top up. Their sfaff are also rude & when i asked when did i buy this recent data ,no one knows all they said is that it doesn't show on their system so how do they know that Iowe them. This is a bad reflection on my credit record, please assist me:(