The entrepreneur (within the limits of the legislation) decides on the contribution base rate

Entrepreneurs have the right to declare any amount – within the limits of the legislation, that is between 60 and 250 percent of the predicted average salary – as the basis for the assessment for social insurance contributions. Exercising this right is entirely up to the employer and the interference of any other entity is not permissible unless it is expressly provided for in the legislation. – Sławomir Paruch and Oskar Kwiatkowski comment for Dziennik Gazeta Prawna.

Article: here.