Compensation for being bumped is one of the few times where you (the passenger) can have your cake and eat it too.

It is unethical to volunteer, get the compensation, and then return the ticket for a refund.

But if the bump is involuntary, I think the passenger has the right to take the compensation and also get a refund for the ticket.

After all, he may miss the engagement at the destination and taking a later flight would be of no use to him. The airline industry has a term for this: "futile flight".

Travel tips:
http://members.aol.com/ajaynejr/travel.htm