Cancellation of contract

I have contract with them, however I lost my job last October...I was working for a labor broker (Quest) under FNB. I alerted them about it because my account was behind with installments, so I have been receiving SMS's from letting me know, and it may affect my credit record. I informed them that I wont be able to continue with the contract!? But they telling to find someone to take over my contract and I cannot do that? Or they said i can supply a retrenchment letter..hence i was hired under an agency, I didn't receive it! So please help me with the issue because i don't want to owe them, and end up having a bad record, which is gonna impact on me getting hired again