Assalamualaikum, I have a debt with a non-muslim, and the owner have gone missing or hard to reach because he live so far away from me, they are in another country as example. How can I pay my debt back? Should I make a sadakah/donation with their name?
I can't imagine that in these modern times you neither have an Adress nor Email nor Phone contact of this person nor know anybody who could act as an intermediate to get any of these. Maybe it is even possible to retrieve this person on the internet. A debt is a due and you must try hard to pay it back before coming with "easy" solutions.
Generally this means you must try to find and locate this person to pay him the debt back. If you tried all possible options and failed you must wait a good while (some months a year whatever seems reasonable for expecting a return of the obligee) maybe your obligee comes back and ask his wealth back. After this time you may do sadaqa on his behalf. If someday your obligee turns back asking about his wealth there still are two possibilities up to his choice (not yours) accepting the sadaqa or insisting on payback.
Note that a non-Muslim can't get rewards for such a deed according the view of at least some scholars.