Is it permissible to place the expiation for an oath (kaffarat al-yamin) in a charitable endowment (waqf khayri) affiliated with a society, with the intention of it being an expiation, especially when one does not know the needy or doubts whether they are truly needy?

It is not permissible to pay the expiation for an oath to endowments (Awqaf). Rather, it must be given to the poor as food or clothing for ten of them. If you do not find the poor, then appoint someone to disburse it on your behalf. If you are unable to appoint someone, then you move to fasting three consecutive days.