Excess fee charge!!?

I have a business account. With fnb. On the 23rd i made an online payment. I had over enough in my account to make payment. Fnb deducted the amount from my balance. Cool. Today i get a charge of R155.00 debited to my acc. They say due to merchant only collecting monies yesterday there was no monies in my acc and they would honour it..oh my word what a **** they then deducted it from my balance on the 23rd. I fail to understand.