Outstanding Balance - Game Accout

On the 14th of January 2020 I went to Game (Cresta Mall) to obtain my outstanding balance seeing that I never receive any formal notifications in this regard via mail or post. A printout noted that I was R519.51 IN CREDIT (I still have proof of the transaction). The following day I went back to Game and made a purchase in the region of R500-00 seeing that I was in credit. The very next day I got a call from Game wanting to know when I will make a payment on my balance of R1000+. With utter shock I explained to them that according to their system I was not owing monies anymore. Since then, I have constantly been harassed by some debt collecting company whom I have explained this situation to time and time again. They even noted that they did detect discrepancy on their side, would investigate the matter and would revert back to ma. It has been 5 months now to no avail. I am refusing to pay the so-called outstanding balance seeing that it is obvious that there are discrepancies in terms of outstanding balances. I am refusing to pay the outstanding balance seeing that obviously the matter was never investigated and how do i know that any amount presented to me now, does exist or is in actual fact correct.