Erroneous charge on account

I would like to once log a complaint over the same issue that fnb has not been able to resolve in over months. In October 2022 I made a withdrawal of R400 at a nedbank atm in Pretoria. It appears fnb processed the transaction late as to my surprise, a few days later my account had a negative balance with the statement showing that I had received a "cash advance". I have no credit at the moment with the bank so I am still trying to figure out why I was made to pay R400 on a legitimate transaction. I doubt very much That FNB is in the business of givng cash advances to people who do not even qualify for such. I have enquiries about this many times, the only response I received was Forty Rands that was paid again to my account to cover the insufficient funds transaction even though I have repeated even over the phone and at the branch that I never received any advance. The withdrawal was cash that was legitimately in my account with I repeat FNB failed to process correctly thus leaving me to pay R400 that was a result of an error on their part