There are two key things to note about redemptions:
If you don't want to a redemption value ie. an amount, to appear on your voucher you can only redeem it once ie. one minute it's active, the next minute it's used - there's no ability to allow partial redemptions.
If you want to allow people to partially redeem their vouchers - ie. use parts of the voucher on multiple occasions - you need to add a redemption value. You don't have to offer part redemptions, you can also redeem the voucher all in one go as well.
This video shows you how you would then redeem either of those voucher scenarios.