Account cancellation

I went to Edgars Westgate Mall asked the lady who assisting me to check the balance for me she said I owe R6000, went to the store again I paid R6000 when she gave me the receipt the was an amount which was R900, I asked how is this possible she said it’s interest. 2 weeks ago I paid R1245 told her to close the account for me cause this is ridiculous she said she can’t I must call the call centre on Monday or send an email. I sent an email and I called but no one is saying anything to me.