As from January 2019, following Council Directive (EU) 2016/1065 of 27 June 2016, new rules regarding taxation of vouchers have been implemented in Poland. The vouchers have been divided into single-purpose vouchers and multi-purpose vouchers.
A voucher will be regarded as a single-purpose voucher when the place of supply of the ultimate goods or services is known at the time the voucher is issued and the voucher can only be used for goods/services at a single rate of VAT. Other than described above vouchers will be deemed as multi-purpose vouchers.
So the answer to this questions depends on the type of voucher. In case of a single-purpose voucher the tax point arises at the time of issue, so before it is used by the client. For multi-purpose vouchers tax point arises at the moment of completion of the transaction for which the voucher is used.