Debit order date.

Hi

I have few contracts with Telkom & they sent me an invoice for December say that they will debit my account on the 24th of December which they didn't do.They then tried to debit on the 27th & found no money in my account because I had already paid the money to them via eft,now they want to charge me for the debit order that didn't go through because they chose to debit on a different date to the one they had stipulated on their very own invoice. I fail to understand why I am being made to pay for their mistake.