How Can FNB allow this

I have had an FNB credit card that was handed over to Barrett and Associates we made an arrangement and I was making payments monthly as arranged untill all payments where completed and the account was closed. They have not debited my account for the past 12 months as the account has been closed, then suddenly this past Saturday they made a demand debit which I was never consulted about and FNB allowed it, How can this happen??