Incompetancy in account management

Easton Berry Inc called me in the early months of October indicating that my account was handed over for collections. They provided me with a settlement amount of R6 063.00. I made a settlement payment to the account on the 13th October 2018. I emailed proof of payment to Sibonelo MV, Nosiphiwe and their Collection Inquiry email. The company called me today to say I need to make a payment to my account. I demanded a letter of settlement which they promised once I send a proof of payment. They have not send me paid up letter, instead they call me and SMS me that I have not made payment. This is absurd.