Product: Credit card or prepaid card

Discover made an unfair judgement towards my dispute ( # XXXX ) against XXXX, and they refuse to correct it and help me get my money back. 
I purchased a discounted XXXX XXXX gift card claimed to have {$1200.00} value at XXXX ( XXXX # XXXX ) and paid {$1000.00} for it with my discover credit card ending in XXXX on XX/XX/XXXX. The gift card was delivered electronically at the same day and XXXX promised a 100-Day Guarantee for gift cards bought from them on their website. Upon receiving the gift card ( Card number : XXXX PIN : XXXX ), I checked the balance at XXXX. Unfortunately, I could not get the balance but just saw a sentence Invalid account number or the account is closed. Then I called XXXX XXXX customer service to figure out this issue. The customer service representative in the gift card department told me through the phone that the gift card was already used on XX/XX/XXXX and the remaining balance was redeemed on XX/XX/XXXX, long before I got it ( The phone call was recorded and is the strongest proof supporting my dispute, however, Discover does not allow uploading voice file. The file is attached here, hope it can be forwarded to them ). 
Instead of initiating a dispute immediately after I knew the gift card history, I emailed the support team on XXXX and got no response in three weeks. Then I submitted a claim ( # XXXX ) on their website stating my issue and requested for a solution on XX/XX/XXXX based on their 100-Day Guarantee policy ( Receipt of the claim is attached ). In their response of receiving my claim, they said the problem should be resolved within up to 21 business days from XX/XX/XXXX. I called the number provided on the email three days after submitting the claim to inquire the investigation process but nobody answered the phone. I kept trying to contact them through phone call and email after that but never got any response. The phone calls were always transferred to voicemail and no one called back yet. And I did not get any email response for my claim with the fact that 21 business days have already passed. 
With the failure of counting on the merchant to solve my problem, I filed a dispute on discover.com for this transaction with some documentations including the transaction receipt, gift card details, gift card history, balance check at neimanmarcus.com et. al. to prove the transaction invalid. The dispute has gone through a few up and downs in the past five months. My dispute was supported at first and got reversed because the merchant uploaded the transactions details, even thought the transaction details just said they sold me the gift card without explaining the most important fact that the gift card was already used before they sent it to me. After receiving the decision, I called the dispute center requesting for a reconsideration with some supplements including gift card inquiry to XXXX XXXX rtfraud department and self statement for this dispute. Unfortunately, my dispute wasnt supported eventually after the reconsideration process. I could not believe this and called Discover to continue the dispute. After reviewing all the documentations the merchant and I provided, the representative admitted that they made a mistake when making the judgement. However, Discover refused to correct their decision and stated they couldn't do anything more for me only if I could get documentation with refund entitled from the merchant even thought I told them the merchant is avoiding my issue and the bank is my last protection. There is no reason I should pay for Discover 's unfair judgement.