Ridiculous #Excess Fee

Time and time again FNB randomly charges excess item fees, as at yesterday our business account was in R257 negative a payment of about R980 was made into the account my available balance this morning is R87??? What is going on FNB R580 in excess fee charges????? My debit orders for the month have already been paid, from my available balance. When purchases are made, the funds are deducted from our available balance even though we can still see the money in balances it is no longer accessible as it has been reserved for the purchase so where do the #excess item fees come from??? Please reverse this fee as it is not acceptable!!!!