Collectioncentre complaint not attending to query lodged 8 March now my credit score is at risk

Last year in October Edgars went over to a new system, I returned R2159.65 worths of clothes and this credit is not showing on my account up and till today! I have been handed over today for debt control due to their system not being updated. At this stage my account is in a credit of +- R1 100. I have mailed *** 5 times now, my first email was 8 March, and still they have not rectify this mistake. So what do i do now to clear my name and get Edgars to wake up and fix my account??