I don't think so, but I would caution this just because the voucher may take some time to get to you and by that time the fare may have gone up.
I used a ticket number to purchase a flight last week that was only $45 but the ticket number had a $96 credit on it. The rep used the $96 ticket number and then told me I would get an email with a voucher number and PIN to use online, but its been over a week and I still haven't gotten it yet.