In the unlikely event NO ONE takes $5000 for a later flight... then offer $6,000...
This is a problem the airline created by intentionally over-booking and insisting their own people had priority over paying customers.
If you caught a cab, and halfway through the ride the cabbie roughs you up and throws you out of the cab because his sister needed a ride would you excuse that?
If its unacceptable for a person to do it, then its unacceptable for a corporation to do it- after all, we have been assured that corporations are people too...