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

I made a purchase off XXXX for {$1800.00} on XX/XX/2020. The item was received in a used state with a damaged delivery box. I began the return process to refund the item as it was not safely useable due to possible shipping damage. XXXX gave me an RMA, and I returned the item in a undamaged and smaller box. After a two week time period, XXXX did not refund me my money. They stated they never received the item. I have three forms of Proof of Delivery from XXXX showing the item was delivered and signed for by XXXX Dock personnel. XXXX instructed me to dispute the charges with my bank/credit card as they would not refund me the money. 
I opened a dispute with the credit card holder, Discover Card, and on XX/XX/XXXX, received a chargeback credit. Due to the fact that XXXX had an open return/refund on that card, the chargeback registered as a refund in XXXX 's system, despite it being a temporary credit. I submitted many documents to Discover Card including the three forms of proof of delivery. Discover card immediately sided with XXXX. We went through three separate investigation 's each time, I received a temporary chargeback credit that was taken away and the funds reapplied to my account the second XXXX replied. XXXX responded every time with a different story. First, the item wasn't received. The second time, the item was the wrong item cause the weight was off by .XXXX. This weight difference taken from the XXXX proof of delivery was caused by the fact that I had to send the item back in a slightly smaller box due to the fact the original was destroyed. The item itself weighed XXXX. Then they stated they had already refunded me on XX/XX/XXXX. Discover Card stated this was not a refund but it was the chargeback credit. The dispute was settled in XXXX 's favor despite my overwhelming evidence that I had returned the proper item and followed all of XXXX 's rules. I tried on multiple occasions to settle the matter with XXXX but they continued to tell me they couldn't find the item. 
Discover Card told me there was nothing more they could do and I would have to pay the charges. So I went to XXXX again and attempted to work a settlement. They stated that since they were in the process of refunding me when I started the dispute, this interrupted their refund process and essentially canceled it. Due to the fact that the card is now associated with a dispute, it is considered a compromised account and they can't send another refund. They also state that they never received a credit back from Discover Card and it shows that they refunded the money to my card on XX/XX/XXXX. Despite that, I still have a charge for {$1800.00}. Discover Card states they processed XXXX 's charges. I have asked four times for them to send me documentation regarding this matter, and I have received nothing. Discover Card refuses to email me any form of proof, insisting it all has to be done by mail. They won't talk to XXXX, and XXXX won't talk to Discover Card. I have been playing middle man since XX/XX/XXXX, trying to resolve an issue within their systems. 
As of now, XXXX has told me they are escalating my case ( This is the tenth time I have been escalated and never contacted via phone or email ). I have no item, and still have a {$1800.00} on my card. 
During the time of all of this, the original card ending in XXXX was flagged for other fraudulent charges by their system and Discover Card immediately sent out a new card with a different number. They stated this would not affect any refunds applied to the old card. 
Discover Card should never have sided with XXXX in this dispute. I gave irrefutable evidence of returning the item. XXXX could never get their story straight. This item was either lost or stolen in the XXXX Nevada Sorting Facility and I am being stuck with the bill.