For involuntary bumps the DOT states:
DOT requires each airline to give all passengers who are bumped involuntarily a written statement describing their rights and explaining how the carrier decides who gets on an oversold flight and who doesn't. Those travelers who don't get to fly are frequently entitled to denied boarding compensation in the form of a check or cash. The amount depends on the price of their ticket and the length of the delay:
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Airlines may offer free tickets or dollar-amount vouchers for future flights in place of a check for denied boarding compensation. However, if you are bumped involuntarily you have the right to insist on a check if that is your preference. Once you cash the check (or accept the free flight), you will probably lose the ability to pursue more money from the airline later on. However, if being bumped costs you more money than the airline will pay you at the airport, you can try to negotiate a higher settlement with their complaint department. If this doesn't work, you usually have 30 days from the date on the check to decide if you want to accept the amount of the check. You are always free to decline the check (e.g., not cash it) and take the airline to court to try to obtain more compensation. DOT's denied boarding regulation spells out the airlines' minimum obligation to people they bump involuntarily.