Not getting what I payed for (Theft)

I am suppose to get 100GB anytime and 100GB night surfer, and not 82GB anytime data, although the night surfer data is correct it is 100GB, this is the longest issue I had with MTN since February, and still not fixed. The best thing to do would be to cancel, since I am not getting what I signed up for. I am paying R999 for this package. I have dealt with the store since February, there is no way that a ticket can be hanging around since Feb, it seems to me the provinsioning cannot be fixed, therefore my contract has been breached from the side of MTN. I think its best to cancel and revert to prepaid which is safer, I don't trust MTN contracts anymore