Vouchers are really not designed to do this. You really want the customer to hang on to the voucher to encourage them to come back and and use their card -- no matter what value remains on the it. The outstanding value is printed on the receipt whenever a gift card is used as a tender.
You will be amazed how many people will hang onto a card or coupon worth 80 cents with your name on it. For this reason you want to order gift cards with your logo, address, website URL, and anything else you think customers might find handy.
If you really want to refund the outstanding value, create a reason code called Gift Card/Voucher Return in Manager | Database, then refund the voucher on a second transaction. You will need to enter the dollar value indicated as the "new balance" from the last transaction receipt (or use the tip below to determine that value).
Setup a reason code:
Manager | Database | Reason Codes | New
Description: Gift Card Return
Type: Return to main inventory
Process the return:
1) In POS press Ctrl-F4 to activate return mode (notice the return mode indicator at top of screen).
2) Enter your GC item lookup code.
3) Select the GC Return reason code.
4) Enter the amount of the return.
TIP: You can view the history and current value on any gift card by pressing Ctrl-Shift-F3 in POS and entering the serial number for that voucher.