Refussal to close the paid up loan

I received a settlement letter on the 09/04/2021 for R181 660.27, Nedbank transferred the R178 360.00 same day. I did an eft on the 12/04/2021 for the balance of R3 300.27. On the 13/04/2021 I called and requested that my account be closed and they should send a paid up letter, the agent told me the account will be closed in the next 5 days. On the 20/04/2021 I called and also emailed as I didn't receive the paid up letter and was told that they have escalated the matter. on the 03/05/2021 I received a statement which shows all the payment but to my surprise there was R840 added, I then called and was told they are still checking my account, I stayed on the line for 15 minutes with no resolution. on the 04/05/2021 I called again and requested my paid up letter the lady assured me that my account will be closed, at 16h20 the guy called and said he is doing a follow up, he explained that the payments do show in my account but I must send the proof of payment for them to close my account. on the 06/05/2021 I receive a new settlement letter with R923. why must I pay this R923 when I settled my account within the 5 days which was stipulated on the settlement statement? I don't owe anything my account was settled, I need the paid up letter. I'm so disappointed with the service I received from you as I've been your client from the previous years. I have rep**** to the mail and attached the proof of payment, kindly close my account.