What is the ruling of Islamic law on increasing the number of days for official assignments to obtain a larger additional wage, knowing that these assignments sometimes require working outside official working hours?

The ruling on increasing working days depends on the authority of the work supervisor. If he is authorized to permit it, then there is no harm in benefiting from what is given in exchange for those extra days and hours. However, if he is not authorized, then his permission holds no weight. One can inquire with the responsible authorities about the work supervisor's authority. One is not obligated to work extra hours or days except for wages, either with permission or by having the issue resolved by the management. There is no obligation to donate labor.