Wrong settlement amount given by FNB

I finally reached a point where I can make a final payment and end my debt review. My debt counselor in ZeroDebt gave me a final amount to be paid to pay back all my debt on 28 January. I paid the outstanding amount on 31 January as agreed. On 10 February I followed up on the status of the paid up letters. I was then told that FNB said that there is another R621,27 outstanding that I need to pay in order for them to close the account. I find this appalling. Why give a settlement amount just to change your mind later? I never ever want anything to do with them.