INCORRECT FUNDS ALLOCATED

I am very annoyed to have received an sms this morning stating my account has been overdrawn. I paid a dentist bill last week Thursday and only received the transaction sms yesterday! I then received an sms today saying my account is now overdrawn. This is impossible as I know I had funds available on my budget facility. The person at the till point even asked me if I would like 3 or 6 months! The straight facility doesn't ask you how many months you want to pay it off over so it was definitely processed on the budget that day. Suddenly today I discover the transaction was processed on the straight. When I called in I was told the funds cannot be moved over and now I must make a payment first to bring my account in line! This is ridiculous as its clearly an internal problem with FNB. How can I be expected to make a payment to fix your error? An this is not the first time FNB has incorrectly allocated funds on my account. This needs to be resolved immediately.