FNB returning debit orders despite having enough money

I am a young restaurateur and have been for FnB for 4 years now, however this is the second time ive had a debit orders returned despite having enough funds??????

Because it's a cash bussiness I have to sometimes bank money in through the atm and then wait for my POS system to bank at midnight to make sure I have enough funds for my debit orders. Last night I had a Vodacom debit order of R 1483 returned, and was charged 100 rand penalty fee despite having had R 1583 in my account. So not just am I being ****** from 100 rand for a transaction that had enough funds for in the first place, I am also getting serious damage to my credit record. This is unacceptable! And to make matters worse, its the second time! After I actively try to not have bounced debit orders