DISCOVERY MADE A MISTAKE NOW I MUST PAY

Yesterday I received a letter dated 03 July 2015, which states that I owe Discovery R 869.60 for a claim that was reversed. I immediately phoned the ***number to find out why. I spoke to a guy (think his name was Nick) and he told me that Discovery paid me too much money and if they could they would have taken it back out of my account, but they are not authorized to do that!!! I received the amounts from Discovery which they will cover for my claim, now they made a mistake and I need to pay them back? This doesn't make sense. I claimed the amounts, which they did not cover, from my GAP cover. The guy couldn't even tell me on which amount they made the mistake. But luckily I don't have to worry, because I don't owe them the money - they will just take it of my next claim. I feel that this is really unfair and not my fault, but now I need to pay them back. I feel that they should make the necessary corrections on their side and not claim the money back from me.