Billing of account

I have a contract with telkom, and my contract end this month, in July they debited and came back to say there was not enough funds in my account yet there was, called them to find out why and consultant insisted that there was not enough funds, I agreed to their statement sent kf R400 something that they will deduct on the 25 of August, to my surprise, on the 26 when I check my statement the only debited R375 and I called to find out why and was told that debit order went off on the 15 and there was not enough that is why they debit less amount,am not happy as I called to confirm with them to debit their full amount on the 25th now I get statement of R650 of out statement amount, yet my contract with them is R135. And my contract must end this month they can even put in place to cancel my contract because of there still outstanding amount, I want to know how do they bill me and why are they debiting my account on the 15 after calling and asking them to debit on the 25, and now am being billed for something I don't understand, even on the statement is not explaining what is R650 for.