I had something similar once. A flight was oversold. I offered to volunteer and agreed on a price with the gate agent. I waited at the gate, to see if everyone would show up. About 10-15 minutes before scheduled departure, the gate agent said you can go ahead and board, and as a courtesy she gave a $100 voucher. (I think I had to check a rollerbag because I was the last to board, which might have contributed to getting the $100).

I got on the plane and took my seat. A minute or two later, that gate agent (or a different one also working the gate) came on board and told me I had to get off. I complied, and I got the negotiated amount (in addition to the $100 voucher). It seemed that they were too quick to try to get the plane boarded and on its way, and the passenger showed up in time after after all.

I did not protest, nor I did ask for more compensation, since I figured that, had I done so, the front-line employee would have gotten into trouble, even though it is clear that they were following the mandate from on high to get planes pushed back on a timely basis, even if it means that some of the time, they cause a situation like this. Delta corporate might not like it when this situation happens, but they clearly like it when planes get pushed back on time.

But I wonder, what if I refused? (This was ~3 years ago, pre-Dao).