Mtn Canal walk gave me wrong info and they must pay

I had a contract with Mtn so I went to pay the balance hence o lost my job. They told me the balance is R12 191 I then paid and I told them that I will continue paying the insurance. So I have been receive airtime and I went at Mtn out side in Canal walk, and they told me that payment was in my account as a credit. So now its finish im must continue to pay the full contract amount. I did not understand because at Mtn in Canal walk next to Clicks that where there told that was my balance and im not working hence I paid the balance. I want Mtn bisses to know that I will not continue with the contract cause they gave me incorrect information. And now they want me to continue paying.