What should one do who bought goods and still owes part of their price, but cannot pay the remaining amount to the seller because he cannot be found, knowing that the seller is non-Muslim?

If the owner of the money (whether Muslim or non-Muslim) cannot be reached, the money should be spent on the welfare of Muslims and in charitable causes, with the intention of giving charity on behalf of its Muslim owner. As for the non-Muslim, no act of worship is valid on his behalf. If the owner of the money appears and demands his money, it must be returned to him. If the person holding the money dies after spending it in the aforementioned manner, he will not be held accountable, due to the Almighty's saying: "So fear Allah as much as you are able."