Unauthorized debit

Hi, yesterday an amount of R704.70 was trasfered from my premier check account to an unknown account that happened to be a loans acc. I have never given loans department to debit on this account nor had any arrangement with them. My loan account was handed over to external lawyers who had been collecting me. How is that FNB sell the account to lawyers and still debit on an account that I never gave them authority to do so, I have recently opened this Premier account and it is not the same account I used when I took out the loan. When I called in yesterday, I was told in order for me to get my refund, I need to send a proof that I have made an arrangement with the lawyers. How so when you guys have no authority to debit. IF FNB does as they please with my money in my accounts, please I authorise to cancel all my products with FNB, from my business account to personal account and also any other savings I have with immediate effect 31/12/2019.