Paid Settlement as Instructed, Now Being Asked to Pay More

I received a settlement offer from Risima Holdings stating that I could settle my account by paying 20% of the outstanding balance. Before making payment, I contacted their call centre to confirm the settlement amount and was advised that the amount would fully settle the account. Based on this confirmation, I made the payment.

After payment was received and when I requested my paid-up letter, Risima Holdings informed me that the settlement offer was allegedly sent in error and that I still owed the remaining balance. This was only communicated after the payment had been made.

I believe it is unfair for a company to honour a settlement offer, accept payment, and then attempt to change the terms afterward due to an internal error. I have requested the call recording and supporting documentation confirming the advice I received but have not yet received a satisfactory resolution.

I am seeking that ALOAN and Risima Holdings honour the settlement communicated and provide my paid-up letter as originally agreed.