There are different approaches to discount an menu item, you can create a promotion or directly discount the menu item price. This article to show you how to discount the value price so your customer can receive additional credits when buying this voucher.
When creating or editing a voucher menu item, you can set the Price to charge the purchaser different to the Voucher balance issued. So to discount a menu item, you just need to adjust the "Price to charge the purchaser" and keep the other as is.
The example below means that your customers only pay $600 and they will receive a $650 voucher.
And this is what your customers will see on the checkout form: