What should I do with the remaining amount from Hajj donations after the pilgrim returned a portion of it because it exceeded his needs, and is it permissible to designate it as an ongoing charity (Sadaqah Jariyah) on behalf of the donors, given that I don't recall their names?

If it is impossible to identify the rightful recipients of a surplus amount from someone's Hajj, then it should be given as charity for the sake of Allah, or for sponsoring orphans, or as an ongoing charity (sadaqah jariyah), because it is money owned by an unknown party, and the ruling concerning it is to give it as charity.