Paid up account, but Ackermans still says I owe R400!!

In October I settled my Ackermans account whish had been outstanding. I settled the entire account, then suddenly a Vincent MANTU from NRB solutions sends me an email stating I'm almost done paying for the account, and that I owe R400. Which is braod day light *******, absolute fruad! How does one settle an account, then a month later be told I owe R400 whereas I have a slip that's clearly indicates Zero rands are outstanding. I settled my account! Whoever Ackermans is in cahoots with in fraduing consumers will need to stop. We already leaving in a country with a very high crime rate. We shouldnt be experiencing such from big companies, how do you even explain to me as a consumer that yes you settled your account, but no you still owe us R400? How? This is diabolical!! I honestly want nothing to do with Ackermans! I need this sorted out! I am definitely not paying any R400! Ackermans will not be getting a cent from me!!