Nedbank Personal Loan - Overpaid

I had a personal loan with Nedbank. I paid off the loan and checked if its paid off but found that i had over paid by R3830.01. The next day i close off the loan account and find additional charges such as Admin Fee R69 and Interest of R2402.40. That now bring the amount to R 1358.61.

My question is how do you charge interest on a paid off account, the admin fee i can try and understand but interest of 2k doesn't make sense. I tried calling into Nedbank for an explanation but no one can explain it and i keep getting transferred from department to department which honestly shows unprofessionalism. I do not think one would refer Nedbank to colleagues or family after this