Complaint about Ackermans

Good day Im Zimkhitha Kandiyeza,I was called by someone from Ackermans on ********** saying Im owing Ackermans R3500,so I dont understand because I paid them all their money at HTN Attorneys they even gave me paid up letter.So I don't understand why they still saying I am owing them. Here is the letter:-

Our Ref : ********** 363 Client Ref : ********** ********** 135523 ZIMKHITHA KANDIYEZA Dear Madam, RE: Tenacity - MATTER NUMBER: ********** 363 / ZIMKHITHA KANDIYEZA : IDENTITY NUMBER ********** 038088 This letter serves as confirmation that the abovementioned account has been paid in full. The final payment was received on 2017/05/12. HTN Attorneys shall not be responsible for any errors and/or omissions should the information contained above be incorrect. HTN Attorneys reserves the right to claim any amount erroneously not reflected, for any reason whatsoever, as and when such amounts become due. Should you require any further information please contact our offices accordingly you