Mixed remuneration system cannot result in lowering the salary
Nothing stops agreeing on a mixed remuneration system in an employment contract. However it is necessary to verify whether any of the adopted rates is less favourable than the others. It is unacceptable that application of one of the rates may lead to the payment of a lower salary than the employee would have received if the other rate had been applied. It should be noted that in this case, the principle of employee privilege, regulated in the Labour Code, is applied, meaning that the solution more favourable to the employee should be used. – Mariusz Maksis comments for Rzeczpospolita.
The article in Polish is avialible here.