As I understand it, when you give a bump voucher to someone else, you give the entire amount to that person. You cant give them 10%, and you keep 90%, or go 50-50, or something like that...they get the entire thing. So whatever is left over after buying a ticket is theirs as well, because you bequeathed them the entire value of that voucher.

The only way I can think of getting around this is instead of accepting one $400 voucher, ask for 4 $100 vouchers. Then you can pass those out, and not lose too much.