Difficulty in closing credit card account

So I have paid my credit card in full. I called them requesting to close the account and I was told of an additional 503 that I must pay. When I requested for this in writing I was told that the amount increases daily. So I asked why it's increasing if my account is paid in full and I was told that it's for interest. My question is interest on which amount since my account is paid up. I said I will do an immediate payment and close it again I was told that it will only reflect in their account after 3 days. I'm confused as to why they are making it difficult for me to close the account. This is so frustrating and disappointing. Someone help me close this account