Over deducted on debit order

Due to financial issues I had to make payment arrangements on my accounts. I stuck to this arrangement as well as increased my payment amount a few months back which I paid diligently every month by EFT. Last month African Bank asked if I could go back on debit order, I agreed on condition that they deducted the arranged amount, I was assured 'they could only deduct what was arranged'. This was the first month for the 'arranged' debit order but they went & deducted the full instalment. I am now left with the tough decision to take this out of our food money or short pay our rent. I am so disheartened, I have no clue how I'm going to get through this month. Needless to say I will be cancelling the debit order.