1. Create a category »Vouchers». Tick the little box «Voucher (no sale / no tax)» at the bottom of the dialog
2. Create one or more products f.ex. »Voucher-20» assigned to the category you created before
3. Inside »Manage Cashy > Settings > Payments» create a manually a new payment method called »Voucher»

If a customer now wants to buy a voucher from you, use the product you created in step 2. The sale of the voucher will appear in your sales reports additionally to the normal shop sale you did. There is no VAT calculated onto the sales from the »Vouchers»-category.

If the customer returns and buys some items from your shop which he likes to pay with the voucher he owns, then use the payment method voucher you created in step 3.If the amount due is higher then the value of the voucher your customer presents, then the remaining amount will be shown in the transaction screen (top left of main screen) and you can close the transaction using one of the other payment methods you defined.

If the amount due is less then the value of the voucher presented, then Cashy-POS will close the transaction and displays you how much you should return in cash to the customer.