Collections on credit balances

22/04/2015 I received a phone call to my place of work and the lady from the Melomed Private Hospital Bellville accounts department advised me that I owe almost an outstanding amount of +/- R700.00. I advised the lady that she has to go back to her records and she will note that I settled the account in June/July 2014 when their offices advised me of the settlement figure. The lady was very determined that I owe these monies and that I am liable for payment. I once again explained that this has been settled and she has to come back to me regarding this before I make any further payments. About 20 minutes later she contacted me with a humble apology that I am actually in credit with an amount of R649.00. After requesting a apology letter as they contacted me at my place of work and I requested interest on my credit to this time I have had no response after even contacting her Manager.