wrongful discount

As I’m about to pay for my dstv on last day of the like I usually do I check for my balance and It was like +R600 mind you I’m on dstv compact I contact them and they are telling me their financial team made a mistake in march or whatever and credited me discount which was wrong and all now I’ve to settle that discount because it was reversed I didn’t ask nor aware of it why are they punishing me for their mistake I don’t have +R600 lying around just for fun