refuse to issue payment arrangment letter

I had a problem pating credit card, but got back on my feet and started paying it off. FNB handed me to Hammond Pole Attorney, I have a payment arrangement with them. I pay diligently, then asked for a payment arrangement letter and was told I need to pay for a few months. I called yesterday and asked for one, after I have diligently paid, and told that they have anow increased my payment by R300 and I have not paid that, so i cant get a letter. So i asked but where is the letter to inform me, you now want R300, this after we have had COVID, but I still pay, they increase it and deny me a payment arrangement letter. I have told them but due to their poor admin, I am now sitting without this letter. They refuse to give me a letter and I am still waiting for admin to give me instruction that they now want extra R300