Is it permissible to take the per diem allowance that was disbursed to a person who did not go on the assignment, and spend it on the secretary's work requirements, or must it be left, knowing that it is registered in his name in the fund?

It is not permissible to take a per diem allowance without performing work. Rather, one must relinquish it and inform the responsible authority. Allah has commanded us to eat from the good things and to do righteous deeds. The Prophet, peace and blessings be upon him, warned that every body nourished by ill-gotten gains is most deserving of the Fire. Sheikh Ibn Baz issued a fatwa stating that taking money without a legitimate right is considered a reprehensible act and a betrayal. What has been received from it must be returned to the state treasury, or given as charity, coupled with sincere repentance.