The hustle to Closing a FNB credit account after settling the amount.

I have settled my credit account with FNB the day my monthly instalment supposed to go through, however it didn't go through, which is a good thing. This is because on the 24th, I transferred the instalment amount to this credit account (early payment). Today after the settling amount transfer, I tried to close off the account, now I am told to pay an additional amount which accounts for the interest incurred (e.g R500.00). I really do not understand why I'm being mislead like this and if everyone is also aware of this. From today, I need all my existing FNB accounts closed, no relationship whatsoever is desired with this company, ANYMORE.