Define a category as «Voucher» and sell vouchers from this category. Selling a voucher will automatically use 0% VAT and their sale will be listed seperatly in the close-reports.

If the customer likes to pay with a voucher he previously purchased, create a payment method «Voucher» and use it for cashy the amount due. If the amount of the transaction is higher then the vouchers value, It is possible combining the payment with an other payment method.