Unhappy about reversed "discount"!!!!

I had an issue with my decoder so I called into DStv, only to realise it was the charger on my decoder that needed to be replaced. I informed DStv that i will be away from home so i will not be able to sort it out so i will cancel my subscription and when I'm home again i will reinstate it... A few months later i reinstated it and realised my account had a credit of +-R500. I called DStv to query and they said it was my discount. I didnt pay that month (this was June) in July I saw I still have a discount and called in again to make sure it was right.... They assured me it was and so I paid the R290 difference that they requested. I kept double checking *120*68584# to make sure I was paying the correct amount. This month I paid my normal R390 and realised that I need to pay an extra R120 odd.... I called in and explained what happened and the lady said it was their error and it will be resolved in 2 days. My ref number was ***5-C1.... 4 days later, no call, no resolution. I received an sms requesting I pay that difference today. I called in again and was told a team leader would call me in 30 mins to an hour and the agent understood my frustration. Its 5 hours later and still nothing. Why tell me it is DStvs fault and all that only to try force me to pay that difference? My issue is that I kept on querying this discount and now they tell me it was an error on their side and I want supposed to get that discount.... But for 2 months nobody informed me this and I had no issues.... DStv what do I need to do to get this resolved??