Refusing to Refund Credit as per Statement

Called the call centre and was given a settlement figure which I paid. Thereafter received an invoice showing that an amount of R 44989.32 was due and they then received a payment of R 45578.17 from me, showing that I have a credit of R 588.85, seeing that I did over pay on my account. Thus the money in credit is due to me but they refuse to pay back and say it is being written off. That is theft.