Unfair Service

I have a problem with FNB.I am using FNB premier,I made an application in April for 3months covid relief for my loan and it was approved after the 26 of April which is the agreed debit date.On the 11 May they refunded me which was month one of the relief.I called on the 23/07/2020 and I was told that this is the last month for the relief as of 26/08/2020 they will start to debit.To my surprise on the 11/08/2020 a debit of 1933.24 went through at the time I didn't have sufficient money for the debit as it came with a minus.I called and communicated with chat support but to date I have got a satisfactory service and on top of that they don't want to accept that its their fault since I also received a message from fnb that reads as follows. FNB: Due to incorrect amount being processed to your account ending ......64379 on 27-JUL-2020, FNB has processed rectifying transactions on 07-AUG-2020. FNB doesn't want to account for this, and to show that its their error the debited amount its not subtracted from the initial amount of the loan. I also requested for a call recording to the covid relief bank assistant who assisted me,I was told call records needs Subpoena.