FNB digs into my account without my permission

On the 18th May 2019 my mortgage protection retrenchment claim was approved and put into place for my FNB bond, yesterday FNB debited my account for my bond payment and obviously I had insufficient money in my account and the cheque account went into a negative.. FNB then had the audacity to go into my FNB savings/investment account and move money from there to settle the negative amount which should of not been debited in the first place as the debit order had to be stopped once the insurance had been approved.. I will take legal action against FNB for doing transactions without my permission or consent between my personal bank accounts. My money is to be reversed immediately and the debit order is to be stopped immediately.