It's an interesting one, especially flying from the EU (or to the EU on an EU airline), where you can get a rebate of up to 75% of the ticket value for an involuntary downgrade or up to €600 for an involuntary denied boarding.

Nowhere near €600 at issue here, but I recently took the train for work and had to reschedule my train. The train I took was $22 cheaper. Here's the catch. The company had booked a nonrefundable ticket. To work around this, the agent gave me a new ticket, plus a $22 "voucher" (not a rebate, but essentially a $22 off coupon) that could be used on a future ticket. I would gladly get reimbursed for the full value and give the voucher to my company, except that it's in my name! In the end I decided to get reimbursed for [cost minus $22] and I guess I'll figure out what to do with the voucher later.

Morally, voluntary bump compensation belongs to the employee.

You see, otherwise employees would not volunteer and some other company's president might not get to fly.

If VDB interfered with work duties then sanctions separate from airline costs would be taken against the employee.