Product: Credit card or prepaid card

Im filing my complaint in regards to my credit card account with Citibank. 
For the last six months Im experiencing extremely difficult time to communicate with Citibank. All my attempts to resolve issue with my account with the above bank were unsuccessful. I have been trying to contact them to resolve issue pertaining to extra interest charged to my account for cash advances. Citibank uses unfair billing practices of charging their customers with extra interest for purchases or cash advance transactions that have been paid off. 
I contacted Citibank Customer Service department on XX/XX/XXXX. The department is referring me to contact Client Focus Team stating that my account is handled by payment arrangement team and they dont have access to my account. They are transferring call to Client Focus Team, that department are not willing to assist me neither, and referring me to Credit Counseling Department, and so on and on.

I spent about two hours being on the phone with Citibank trying to speak to Account Manager. After numerous attempts to request to speak to Account Manager, I was transferred back to Customer Service Department and spoke to Manager named XXXX. He was reviewing my account with me and confirmed to me as a previous representatives that my cash advance balance that was posted to my account over 2 years ago, was not paid off. His conclusion based on the payment rules allocation of Citibank. XXXX stated to me because I was making minimum payments towards my balance, all my payments they were distributed towards my purchases with low APR and excess of payments will be distributed towards to higher APR which is cash advances. On XX/XX/XXXX Citibank decided to move my cash balance that is subject to interest rate of 9.9 % to my standard purchases to eliminate any interest they charge me for cash advance every month. Technically they did not resolve my dispute with extra cash advance interest they charged me since XX/XX/XXXX. They motivate their decision as a courtesy to show me that there is no more interest charged to my account towards cash advances. 
Im completely disagree with their decision about cash advances and their payment allocation rules. I conducted my own research of the interest that were charged to my account since XX/XX/XXXX. Billing statements show the same balance of {$66.00} in average subject to interest. Regardless of my payments that I made towards my outstanding balance, were not applied to cash advance transaction.

The cash advance transaction was placed on my account in XX/XX/XXXX for {$54.00}. Prior to my account was closed, the APR for cash advances was 25.74 % and I was making more than minimum payments. Citibank states that all my payments were allocated towards my purchases, and not towards my cash advance transaction. In XX/XX/XXXX my account was enrolled into Debt Management Program and my APR for purchases and cash advances was changed to 9.9 %. There are no more purchases made to my account since account was closed. Citibank is playing game and uses their unfair billing practices and charges me extra interest towards cash advance transaction that have been paid off. 

All my attempts to discuss my account with Citibank did not bring me any positive results. It is not possible that the small transaction of {$54.00} was not paid off. 
Citibank does not admit their error and they motivating their response there is no error from their side. I was charged extra interest from them and now since they moved cash advance balance to my standard purchases, it even harder for me to prove that cash advance balance on my account was paid off.