The rules for issuing invoices in Poland are specified in Art. 106a-106q of the Polish VAT Act (Journal of Laws of 2023, item 1570, as amended). The VAT regulations do not contain any provisions requiring Polish taxpayers to issue invoices only in Polish currency so in fact an invoice can be issued in any currency no matter what is the type of a given transaction. In art. 106e section 11 of the Act only states that tax amounts are shown in PLN. Therefore, when issuing an invoice in a foreign currency, that the tax amount should also be expressed in Polish zlotys.
Source: polishtax.com
