The logic behind this question is, that a voucher is a form of payment that is typically purchased earlier from the same venue. Like a Gift Card, or a Discount Booklet (eg, buy a book of 10 coffee vouchers at a price of eight coffees). You have already received the money (thru the POS) from the sale of the Booklet/Voucher, so if you receive a voucher for payment it should not be added to the Sales figures, but recorded for reconciliation purposes.
What do you think ? I would like to hear any views on this.