Misled by vodacom and promised a lie.

I have been paying 650 rand on my vodacom account. I then decided to make an additional payment of 1000 rand and asked for settlement amount then I did i was informed I can instead reduce my monthly installments to 400 rand and the fee to change will be 1000 rand. Instead my installments are reduced to 611 rand. I have been mislead by vodacom. In trying to resolve this I called the contact center and they keep taking me from pillar to past and nothing is resolved. How can I pay 1000 rand to only reduce my installments by less than 40 rand. It makes no sense. I would have continued and paying 650 rand and adding that 1000 rand as additional payment.