Debit date issues

Paid my owing amount using EFT because Vodacom did not debit on the 25th (Saturday) and had attempted to debit on the 27th. I explained I had paid them and the reason the debit order did not go through was there wasn't enough funds anymore in that account. I still got charged the R100 fee for a rejected debit order and they refuse to refund the charge even though it was their fault the debit order didn't go through