Bad Service

I have a loan from FNB which was handed over to Barrett & Associates, I have been making payments regularly, recently they made an offer for a discount if I pay off the loan within a specific term, I then confirmed to them via email that I would be paying off the loan within 3 months, after 2 instalments payed, I am left with one payment due on the 25th Dec 2020, my issue is that I have called them several times requesting a letter which will be proof that I have entered into this agreement with them, every time I am promised that I will be receiving an email, it has been 2 weeks now, but still no email. The agent in charge of the account is Thobeka, who every time keeps on telling me that she has sent the request to her manager to send me the letter. I am worried that as I am about to make the last payment, getting proof of payment or even the account being closed will be a problem. It is very discouraging when you are trying to get your credit profile to be in good condition but you get Creditors dragging their feet as though they prefer for you to remain in debt forever.