Is it permissible for me to take a commission or gratuity for additional tasks assigned to me compulsorily, which are unrelated to my primary job as an accountant, and without the sponsor's knowledge?

An employer is not permitted to compel an employee to do something that was not agreed upon, due to Allah’s saying: (O YOU WHO HAVE BELIEVED, FULFILL [ALL] CONTRACTS) and the Prophet’s (peace be upon him) saying: (Muslims are bound by their stipulations). The employee has the right to refuse or to demand additional pay. If the employer refuses, then the employee should leave the work or refer the matter to the competent authorities.

As for commissions, if they are from the sponsor’s money, they are not permissible without his knowledge, because that constitutes deception and treachery. If they are from clients, they are forbidden gifts to employees.