Dissatisfied fnb client

I paid my loan in full on the 21st of June 2023 and it showed a zero balance. I was surprised that fnb went and deducted R681 on the 23rd of June. Now to reverse that, they want to charge me R45 as they say I'm in breach of contract. The contract is null and void now as this account is paid up. Please reverse my money and send me a paid up letter