No, sales of beer vouchers on the day of the festival must be registered as other payments. The actual sale of beer happens when the vouchers are exchanged for beer. The sales receipt must be printed at the time of redeeming the voucher, where the means of payment is reported as "beer voucher" or similar.
This results in a change of earlier accepted practice, where we have expressed that sales of beer may be registered as a (cash) sale at the time of selling the beer vouchers. The new cash register legislation enjoins the requirements to print out receipts at the time the goods/services are delivered/"passes the point of sale". This must also apply at festivals.
If the customer does not have a right to have money refunded for unused vouchers, the value of unused vouchers by the end of the festival must be registered as beer revenue liable to VAT.