Product: Checking or savings account

Citibank has charged fees on my closed checking account that I have initiated to close on XX/XX/2019 and confirmed by the Citibank customer service representative that it is closed. I received a letter from Citibank on XX/XX/2019 stating that there is an unpaid balance of {$61.00} in my closed checking account that I need to pay off, else they will report this to credit bureaus. I am not aware or sure why is there an unpaid balance on a closed account. 
Following is the background of this issue : On XX/XX/2019, I have transferred all my money in the account to an external account and the transaction was completed. I confirmed it by logging on into my Citibank accounts and my external account to confirm the fund was successfully transferred to my external account on XX/XX/2019. 
On XX/XX/2019 I called to Citibank to initiate to close all of my accounts and the customer service representative checked the account and confirmed that both the checking and saving accounts are all clear and can be closed. 
Then on the beginning of XXXX, I received a letter from Citibank stating that there is unpaid balance in my checking account. I was shocked because as far as I know both of the checking and saving accounts were closed in XXXX with XXXX balances and was cleared by them. I have also made a note for myself on when the accounts were closed XX/XX/2019. 
On XX/XX/2019, I called to Citibank customer service to find out more details about the letter and the unpaid balance. I was told by the Citibank representative that both the checking and saving accounts are closed and she told me I am all set and there is nothing to worry. But I insisted her to look into it further because I received this letter stating that there is an unpaid balance due and it will be reported to credit bureaus. She kept on telling me that I don't have to worry because both accounts are closed on XX/XX/2019. That made me worry even more because all of the accounts were closed on XX/XX/2019 and not XX/XX/2019 as she told me. I insisted her to look into further so she put me on hold for a really long time - finally she got back and told me that the checking account was charged for a " below balance fee '' and " overdrawn fees '' for a total of {$61.00}. Now she changed her tone and she told that I now have to pay the balance. 
I told her I closed the account on XX/XX/2019 but then she said it is my responsibility to ensure there is no outstanding transaction when I closed the account. I told her that there was no outstanding transactions or balances when I closed the account on XX/XX/2019. I have told her the customer representative who had helped me then had also checked and ensured that everything was good and closed the account on XX/XX/2019 ; But she insisted that it was my responsibility not Citibank, and I have to pay the fees! 
As a good faith customer - I have done my due diligence to ensure there is no outstanding transaction when I had reached out to close the account with Citibank on XX/XX/2019. And it was confirmed by Citibank Customer Representative to me on that day that all of my accounts are in good status and it can be closed. 
I have done my best to explain to the Citibank customer representative on XX/XX/2019 call, but the that customer service representative refused to remove the unfair and incorrect charges and stating that since the account was closed that she can not do anything ; But interesting enough how can they charge fees on closed accounts but ironically they can't remove incorrect charges? Another suspicious thing is that Citibank reported that the account was closed on XX/XX/2019 instead of XX/XX/2019. Another thing that doesn't make sense is that if they couldn't close the account on XX/XX/2019 because of " outstanding transactions '' then how can they close it on XX/XX/2019 since there is still " unpaid balance '' on the account? All of these indicate that Citibank is trying to impose inaccurate charge on customer banking account by postponing account closures. This is really an unfair and unethical practice and it should be stopped. 
I am seeking your assistance in resolving this issue by requesting Citibank to remove the incorrect charges of {$61.00} from tied to my closed account and reassure that this won't impact any of my credit record going forward. If you need any additional information, please let me know. I appreciate your help on this.