Craz is correct. Different kinds of vouchers work in different ways. For an ecert - they are one time use - when you buy the ticket, the value is gone, and the ticket value is going to be considered only the amount that you paid in dollars. There are others, like the old UA type B vouchers, where the full amount of the fare was shown, with the amount off showing as a discount. For these, if you canceled, you could receive the entire value back in credit.