over charged

i requested a settlenment on the 23/02/2016,paid it in on the 24/02/2016 and on the same day i asked for the account to be closed and sent a letter. i was told to go request it at the branch as its done there, on the 25/2/2016 when i went to the bank i was told there's intrest generated already on my account i must pay that first before they can close the account. how can i pay interest on a settled account, interest was supposed to be added on when i requested the settlement on after.