What is the ruling on overtime work, given the possibility of informing the direct supervisor, and is it obligatory to give up part of the salary in exchange for this work?

It is permissible for an employee to work for himself outside of official working hours, unless the employer stipulated that he not work for others. If the contract stipulates that it is not permissible to work [for others], there is no objection to it if authorized by the empowered official.