A voucher is not an IOU. An IOU is a promise to pay later. A voucher has a cash value and the person who is paid with the voucher just has to redeem it for the value. A coupon could be considered a voucher.
A voucher is a paper, sort of like a coupon, that entitles you to something. A voucher has a specific value and is redeemed for a specific product or service.
