interest charged for no reason

I made a payment of R11042.00 on the 17/09/2018 to settle my smart loan account at FNB. Payment was from Capitec to FNB. Before making this payment I visited FNB bank city branch to get my settlement amount. They put me on the phone and I was told to check balance on my app. I asked them is that all I need to pay, they said yes. I was then assured that there will be no debit order from my account on my normal debit order date. To my surprise, my bank account was debited with R2000.00. I called to get a refund and my account was only refunded with R1700.00. I enquired about this from your contact centre and I was advised that I was charged for interest. I would like to know which interest was I charged for if my account was settled on the 17/09/2018 with an amount that I was given by you guys. How come I'm charged interest 2 times in one months and interest was charged on R0.00. I want my money back please