Bothered by calls a year after settling an account.

In May 2017 I settled my Absa credit card. Absa then called me and told me that my account was in credit and they needed to pay me back the credit. They paid back R2500 from the settlement but since February this year I have been receiving calls from their collection lawyers Daly in Durban asking me to pay R900 to Absa. I have never used the credit card since settlement. Why am I now being asked to pay R900.