Lars Bauer, federal law requires airlines to follow a certain protocol when overselling flights. That protocol includes compensation: "Compensation varies by how long the passenger will be delayed. If the airline can rebook the passenger and get him to his destination within an hour of his originally scheduled arrival time, no compensation is required.

If the passenger will arrive between one and two hours later than planned - or between one and four hours for an international flight - the airline must pay the passenger twice the amount of the one-way fare to his destination, up to $675 (£550).

If the passenger will be delayed more than two hours - or four hours for international flights - the airline must pay him four times the one-way fare, up to $1,350." From http://www.telegraph.co.uk/news/2017/04/10/united-airlines-incident-rules-airlines-bumping-passengers-overselling/