I have some trips coming up that I am hoping to get bumped on. If I am able to convince the gate agent to give me the $$$ voucher instead of the DBC-Free one, how does it work if the flight you book is less than the voucher?

I.e. The voucher is for $400, and your trip is $300. Do you get a $100 voucher to cover the difference, or do you lose the extra $$?

I cannot seem to find the answer on the website or by searching the forum.

Thanks.