You have two options. You can send a "Request for Information" or "Retrieval request" through your processor to request a copy of the receipt or you could try contacting the bar owner directly to see if you can get a copy.
Couple of possibilities here:
1. The customer was too drunk to remember spending that much.
2. An unscrupulous friend stole grabbed the card without your customer knowing and put it back.
3. An unscrupulous employee swiped the card while processing the authorized payment to help themselves you some of your customer's money.
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