Suppose I find lost property, and can return it to the owner. Am I required to return the object that was actually lost, or can I give the owner a duplicate? (And why?).
I am adding an additional assumption that the item has some identifying mark that would more likely obligate you to return it. According to most opinions, an item have no identifiable markings does not need to be returned. However, if you know exactly who the owner is, you MAY need to return it, anyway, in all circumstances.
Having said this, based on what I gleaned from this jlaw.com article there is a debate.
However, both Jewish and common law accept that although one does not acquire title to the object, one is not completely precluded from using it. There are two schools of thought on this topic within Jewish law. Most authorities, following the rule of Rabbi Shabtai Meir HaCohen, state that in situations where Jewish law precludes the finder from taking title and the law states that the "the object should reside until Elijah comes," that phrase is properly understood to mean "the object should reside in the finder's pocket until Elijah comes" - while the user has no ownership rights to the object, it is his to use and derive benefit. Should the finder break it or wear it out, he owes the owner the value of the object, if the owner comes forward and claim it. This seems to be the approach of most decisors. Other Jewish law authorities disagree, and rule that in situations where the owner does not come forward, the object resides in peace, without anyone authorized to use it for his own benefit. According to neither of these schools of thought may the possessor (who is not the true owner) legally transmit valid title to another.
I have excerpted what I thought was the most relevant paragraph to your question. There are many other factors involved. However, I inferred that if you decided to exchange the lost item, it means that you have possessed ownership of the item.
The article is long, but it is quite interesting reading.
The source of the mitzvah is Dvorim 22 (1-3).
- You shall not see your brother's ox or sheep straying, and ignore them. [Rather,] you shall return them to your brother.
- But if your brother is not near you, or if you do not know him, you shall bring it into your house, and it shall be with you until your brother seeks it out, whereupon you shall return it to him.
- So shall you do with his donkey, and so shall you do with his garment, and so shall you do with any lost article of your brother which he has lost and you have found. You shall not ignore [it].
Rashi on possuk 2 says
whereupon you shall return it to him: That there shall be something left in it to return, that it should not consume its [whole] value in your house, so you should claim it from him [from the owner]. From here, [the Rabbis] said: Any animal that works and eats, should work and eat [the proceeds of its work]; and [any animal] that does not work, yet still eats, should be sold [by the finder, and the money restored to the owner]. — [B.M. 28b]
So it seems that the ideal is to return the actual object. If the object cannot be returned (in Rashi for economic reasons) then what remains of its value can be returned.
This implies that substitution is not a valid option. But I do not have a source that states that explicitly.
There is a well known principle in Gemoro that: (My translation (and understanding) of the link):
אדם רוצה בקב שלו מתשעה קבין של חבירו See Bovo Metzia 38a
Man wants his one measure (of produce) more than the nine measures of his friend
The gemoro explains that the nature of man is, that he prefers to have a single thing created by his pains, than nine measures created by his fellow's efforts.
Consider a case where someone deposits fruit with his friend. The friend sees the fruit starting to rot or be eaten by mice. This rule is presented as a reason why the friend should not sell the fruit. For, though in purely economic terms it is better to sell the fruit, and with the money he receives he could buy new fruits, we assume the depositor wants his measure rather than nine measures which his friend might give him.
It should be noted that the principle is an exaggeration, as in the Gemara we find that if a certain percentage of fruit rot, a minimum ratio of nine to one, then the friend should sell the fruit in Beis Din to prevent a loss of depositor.
The principle of “Man wants his one measure (of produce) more than the nine measures of his friend” would exclude giving the owner a duplicate.