Day light *******

Can someone please explain why FNB keeps *****ing our money & running our accounts on a minus & blames it on pending transactions that did not take from the account but each time we purchase our balance deducts whatever it is we bought. Eg: if your balance was R900 & you purchase something that costs R400 then the balance is R500. But somehow with FNB they claim that the transaction is pending waiting for the store to bank its money & the transaction has not gone off your account but yet the available balance has changed. Then when their so called pending transaction do go off & there is R400 left in your account then it runs your balance in - R100 because the initial R500 was not available but when you made a purchase the transaction went off your balance. That is total bull ****.