Deducting money from account every week

My husband took out a personal loan through nedbank a year ago. The installment is set to go off by debit order on the last day of the month, and it has been for the past year. The payment has never ever been behind. Every Friday money gets paid into his account, and every Friday they deduct the installment from that money, even though the loan has been paid and is up to date. He has spoken to numerous people at nedbank, and they have all assured him that it was an error, the loan is not behind and the money will be paid back. So every Friday after speaking to them, the money is paid back into his account. This has been going on for weeks now. This morning he got an email from nedbank saying that the payments have been insufficient, because he had to reverse the payment again on Friday due to the fact that the loan was paid at the end of the month as it always is. This is pathetic. He has spoken to so many people, and everyone said it's just an error and they will fix it. How long do we have to fight for this to be fixed??? And their ***** up keeps costing us money due to the bank fees