What should I do about a debt that I was unable to return to its owner, whose whereabouts I do not know, and who may have passed away?

If you are unable to deliver the rent to its owner or her heirs, you may donate it on her behalf. If the owner or her heirs appear later, and they approve of the charity, then its reward will be yours. Otherwise, you must pay them the rent. It is permissible to donate trusts and funds whose owners are unknown, provided that their value is guaranteed to the owner, as this benefits both the giver and the recipient.