Collection of Netstar account

My account was handed over to them at R1600. I explained to them that I had already paid R1000 to Altech. They requested I send them proof of payment, which I did. They went quiet for a few months, now they are demanding R1900, having app**** interest. I said to them just get the right amount from Altech Netstar, then I will pay them R1000, the difference will be a tip for them.