Complaint sub-issue: Credit card company isn't resolving a dispute about a purchase on your statement

On XX/XX/2019 I made a cash payment in the amount of {$690.00} upon finding out from the merchant that I can use cash rather than my credit card. Unfortunately, the trainee at the location held my card prior and ran it though. I was assured that it would fall off the card as I paid cash. 
After the time, it did not fall off and I was charged on my citi card as well. I disputed the charge and they did an investigation. They asked me for documentations of what happened. The problem was that the card was hit and should have just been removed. When I went home that day, the credit card showed pending, and i was assured it would fall off. I reviewed my cash receipt and realized that I was charged for things that I didn't agree too and immediately called the merchants corporate office. He said for all of my troubles, he could provide me with my cash refund of {$580.00}, I could pick it up at the branch or he could send a paper check. I did not feel comfortable going to the location and wanted the instant funds returned in case the check would never arrive. He said that I can put it on any card as it is my cash refund. I decided that I would pay my citi card so that I could pay it down. 
I waited 4 more days and logged into my account, the transaction that was to drop/fall off now hit the account. Citi immediately assumed that the cash that was returned onto my card was for the pending, now non pending charge. I was instructed to get a letter from the merchant explaining this. I sent it to them. They still did not understand what was going on. A simple charge that should have went from pending to removed. So I spoke to managers and supervisors at citi ... they assured me that they would work with me as I am protected. One manager said to obtain a good faith letter from the merchant, this was on XX/XX/2019, almost a year later!! I, with more running around & getting the runarounds, obtained another letter from the merchant explaining that the {$580.00} credit was not to be touched as that was my CASH that I applied to my card as a payment on my account and that the {$690.00} is still owed fully to me as it should have been an easy double charge wipe by citi. I called to follow up to make sure the letter was rec 'd by citi. Again, I spoke to a supervisor and was assured that with the good faith, if the merchant doesn't respond within 31 days, XX/XX/2019, they ( citi ) would take the full {$690.00} away from my card balance. 
I called a few times in between to make sure this was still going to happen. Every time I called in, the rep would say " I am sending this to our investigator to make sure an update is provided asap '' & every time i would hear nothing. Now it is close to XX/XX/XXXX, I figured I would call XX/XX/XXXX, and was told there is nothing that can be done. After almost a year, I am being told nothing can be done. Why so many different promises & working to get the items needed for a simple fix. It should have been handled right away, but no one can understand what is going on over there ... & they handle our money. I don't trust them with any of my transactions anymore. I asked them what all the running around was for that I had to do and all of the follow ups. I was assured multiple times by reps and managers that this would be resolved as I am protected as a citi card holder with a good faith letter. It is almost a year later & I am still getting runarounds.