I heard it myself back in late June. Prepaid AMEX gift card. I was on my one great team pass to Prague from JFK when apparently the AA flight that night had cancelled. Our flight somehow in an hour went from 5 empty seats to oversold.
The CZ Pilsner beer festival was the next day and we had already been upgraded to first for the flight, I was already tasting some fresh pilsner.
The gate agent said they were now oversold by up to 12 people and the bidding started at $200 AMEX gift card, car service to/from manhattan with hotel, and upgrade to First the next day.
I kept my head down and boarded when first class boarded. By that time the amount of the gift card offer had gone to $600, then $800, and then $1200 even before I boarded. I was amazed! No idea what the final amount they got to before they boarded coach.

Delta dollars were never even mentioned, right to the AMEX gift cards.

They could have had me for $400, but I'm sure had I raised my hand they'd said, well you can just give your pass away but were not handing an employee any amex card.

It was great beer though!