Product: Credit card

On XX/XX/2023 I utilized a promotional check from Discover, which I had done before, to transfer a balance of about {$2400.00} from XXXX XXXX over to them for 1 year at a 0 % interest rate. I have never paid any interest on a credit card nor have I ever had the need for a cash advance. I have worked for three banks myself and am very familiar with the industry and how everything works. 
I set up my auto pay immediately for {$200.00} a month with Discover and do not need to check my account monthly as I have the autopay and I do not use the card for purchases. I was about to transfer a small balance for dental work over to Discover when I noticed the balance was still quite high. I looked at a statement and noticed {$55.00} of interest had been charged so I immediately called to find out what had happened. What they proceeded to tell me is so outrageous and fraudulent, I am still in shock. I expected to hear that maybe I had confused the length of the promotion, which is not likely with how diligent I am with my finances, but they went on to tell me that I had been charged interest for a CASH ADVANCE. I asked them how that was even possible when I had used their own Discover card check with the balance transfer rate posted on it to pay off the balance with XXXX XXXX and transfer over to them. She said she understood perfectly that I had used the correct check, it was for a balance transfer for a year for 0 % but ... .here 's where the insanity begins .... '' XXXX XXXX cashed it as a cash advance, so it was posted and charged as a cash advance by Discover because of XXXX XXXX. '' This makes ZERO sense in every capacity imaginable. They acknowledged the check was for a balance transfer, but it was XXXX XXXX 's " fault '' for cashing it as a cash advance. This is not how a cash advance works at all. Discover told me they would not refund the interest they had fraudulently been charging me this whole time, and that I was out of luck. I asked them how in the world this was a cash advance, and she would not answer me, just kept casting blame on XXXX XXXX. I asked them to explain to me what a cash advance was and how this qualified as one when this was an existing BALANCE, not new money being issued to me by anyone. i never agreed to this, this was not the offer or what the check was for. They would not answer my questions. Yes, I got irate because they were refusing to answer my questions or take responsibility for THEIR mistake. I asked how in the world they could charge me for a cash advance when no new money was issued to me or without my knowledge or consent, and again they blamed XXXX XXXX. First of all, XXXX XXXX simply accepted payment and transferred the balance over to Discover. They had NO IDEA what the heck Discover was even trying to say by claiming they " cashed it '' as a cash advance when they simply accepted payment by check. There would be no penalty nor any kind of labeling of a cash advance on a payment! They were RECEIVING payment, not issuing money and they also pointed out that Discover collecting interest on money they did not issue was fraudulent. How could they claim it was a cash advance when they NEVER issued me any money, they simply accepted a balance transfer, knew and admit it was a balance transfer, but refuse to honor it. I called my other two banks and they said what they did is completely fraudulent and they owe me my interest back. I have lost {$410.00} and have no intention of paying off that part of the balance when i close the account tomorrow. I will provide anything if requested but am having a hard time getting anything from Discover to defend why they fraudulently charged me as a cash advance.