Arrangement Letter

I have a personal loan with Nedbank, that unfortunately fell into arrears of 5 months at the most. I then called their legal dept and made an arrangement to repay R3000 monthly towards the arrears and increase as time goes. Now the issue is that, they refuse to give me proof of this arrangement in writing or confirmation letter of the agreed amount. As i sit, i have no proof of what was agreed? I dont know whether its agreed to? I have nothing to stop any further legal action? I don't know whether they will deduct agreed amount or not, because there is nothing to confirm such, on paper.

What is so difficult to hand me confirmation letter on what we have agreed to?