A father vowed that if his son occupied a certain post, he would slaughter a sheep for the sake of Allaah Almighty, but he cannot fulfill this vow. Can the son fulfill this vow instead of his father, as he is capable to fulfill it? In this case, is his father considered sinful?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Slaughtering a sheep for the sake of Allaah Almighty is considered an act of obedience, and getting a job is a permissible matter. It is obligatory to fulfill a vow that is conditional on the occurrence of  a permissible matter. Thus, the vow-maker has to fulfill his vow. If he is unable too, then he should offer an expiation like that of breaking an oath. If he permits his son or anyone else to fulfill the vow on his behalf, then this clears his liability.

If his son or anyone else fulfills the vow without his permission, then this does not clear his liability because it is an act of worship. Acts of worship in which deputizing is accepted, like fulfilling a vow, require an intention or the delegation of others.

Allaah Knows best.