Is it permissible to take money from the employer without his knowledge, due to his underestimation of the effort exerted, his exploitation of the worker's need, and his failure to give him the rightful wage?

Determining the wage is a condition for the validity of the ijarah (leasing/hiring contract), and not specifying it invalidates the contract. In such a case, the hired person is entitled to a fair wage (ajr al-mithl) according to the majority of scholars. One must cease work until a specific wage is agreed upon. For work already performed without a specified wage, a fair wage is due. It is not permissible to take extra money beyond the agreed-upon wage or the fair wage. It is permissible to take from the employer's money without his knowledge if he withholds the wage and it cannot be collected by other means (the issue of al-Zafar).