Overcharged but cant get my refund

My daughter took a month to month contract for data. Her internship came to an end. I paid the acc off for it to be canceled. They gave me amount to pay off I did. After that they said I overpaid by R109.00 when I try to reclaim it as a refund they refused as they do not do refunds. The acc must be re activated so that they can give me any product worth that " ONLY" R100 as per their consultant. At Festival mall Kempton Park.