Worst customer service

I recently wrote a review about dstv I cancel a contract with them end April as I was no longer working, they r(multichoice) responded to my review the same day after it was published,I had a telephonic conversation with one of their agents I requested her to send me a settlement letter so that I cancel the contract for good. After setling the account I sent them proof of payment as per settlement contract. I did that today to my surprise they say I now own them R621.I am really ****ed off I know after paying this amount ,they also talk about(terms and conditions)I tried to hustle to pay the outstanding amount as per contract. I don't know how am going to pay this amount. I am really disgusted and frustrated.