Gemara

Version #1 (Neharda'ei i.e. Chachamim of Nehardai): One cannot write a Harsha'ah (power of attorney) for Metaltelim (movable objects).

(Ameimar): This is due to R. Yochanan's law (since it is not in the owner's Reshus, he cannot authorize another about it).

(R. Yochanan): If something was stolen, and the owner did not despair, the thief cannot make it Hekdesh, for it is not his. Also the owner cannot be Makdish it, for it is not in his Reshus (under his control)!

Version #2 (Neharda'ei): One may not write a Harsha'ah for Metaltelim that were denied.

Inference: This is only because it looks like Sheker. If they were not denied, one may write it!

104b: Rav Papa was Masik (owed) 12,000 Zuz in Bei Chuzai. He gave it to Rav Shmuel bar Aba, Agav the corner of his house, who then brought to him the money.

Bava Basra 127a (Rav Papa): If it was once known which son was the firstborn, and then it was forgotten, one writes a Harsha'ah to the other (and they split the extra share).

Rishonim

Rif (Bava Kama 27a): The Halachah follows R. Yochanan and Version #2 of Neharda'ei. If a Harsha'ah was written on denied Metaltelim, we do not force the Nitva to go to Din or swear based on it.

Rosh (7:4): R. Tam says that a Harsha'ah makes a Shali'ach. Even though he cannot be Makneh the loan, he makes a Shali'ach. The episode with Rav Papa supports this. It says he was Masik 12,000 Zuz, which connotes that it was a loan. Another support that Harsha'ah does not depend on Kinyan is that we hold like Version #2 of Neharda'ei, that we write a Harsha'ah on something stolen, even though one cannot be Makneh it, for we hold like R. Yochanan. We must say that it was enacted that Shelichus works as if the Shali'ach himself acquires.

Rambam (Hilchos Sheluchim 3:7): If one wanted to be Marsheh a deposit of coins, he is Makneh the coins Agav the land. Then, the Shali'ach can go to Din with the Shomer and bring them. One cannot write a Harsha'ah for a loan. This is the law according to the Gemara. The Ge'onim enacted to write a Harsha'ah for a loan, lest people borrow money and go abroad. They also enacted that if one was Marsheh to take money with another, or to claim a loan and the Makneh had no land, he may be Makneh four Amos of his share in Eretz Yisrael, and be Makneh the coins Agav the land. These are extremely flimsy and shaky matters. Who says that he has a share in Eretz Yisrael? Even if he does, it is not in his Reshus! The Ge'onim who enacted this said that we do not say 'the law will pierce the mountain'. It is merely to threaten the Nitva (defendant). If he wants to judge and give according to this Harsha'ah, he is exempt, for the one with the Harsha'ah is no worse than a Shali'ach appointed with witnesses. We do not force the Nitva to go to Din or swear based on this Harsha'ah if he does not want to. They also ruled that if one had a loan, whether with a document or with witnesses of a Kinyan, we write a Harsha'ah on it, for this is denial of a lien on land. They did not enact for a Milveh Al Peh that he denied.

Ra'avad: This is if he did not give over the document.

Kesef Mishneh and Beis Yosef (CM 123 DH u'Mah she'Chosav b'Shem): 'These are extremely flimsy and shaky matters' refers to being Makneh Agav one's four Amos in Eretz Yisrael. The enactment to write a Harsha'ah for a loan, lest people borrow money and go abroad, was proper. This concern applies also to a Milveh Al Peh! Presumably, the Rambam means that letter of the law we do not write a Harsha'ah on a Milveh Al Peh, for it has no lien on land, but the enactment allows even this. However, his words connote otherwise. Nowadays the custom is to write a Harsha'ah even on a Milveh Al Peh that was denied, for Tikun ha'Olam. How does Agav help for a loan? Chalipin should suffice! Perhaps since Chachamim were lenient to say that Harsha'ah helps for a loan, they were stringent to require Agav.

Hagahos Maimoniyos (30): This is flimsy, for the Gemara says that it is possible that someone has no land to be Makneh Agav land (Bava Kama 46a). Why can one make a second Harsha'ah? He already gave away his four Amos to the first Shali'ach! R. Tam says that it works through his admission that he has land and is Makneh to the Shali'ach. Even a false admission acquires (Bava Basra 149a).

Poskim

Shulchan Aruch (CM 123:1): We do not write a Harsha'ah for a deposit that the Shomer denies, nor for a Milveh Al Peh (a loan without a document).

Shach (1): This is the Rambam's opinion, that Harsha'ah needs a proper Kinyan. We hold like the other opinion, that even if something cannot be acquired, Chachamim made a Shali'ach as if he acquired it.

Question (Tosfos 70a DH Lo): Rashi says that since one cannot be Makneh what is not in his Reshus, if it was lost, the owner could claim it again from the Shomer. This is difficult, for he is no worse than a Shali'ach. If the owner said to give it to a Shali'ach and the Shomer did so, he is exempt.

Answer (Bedek ha'Bayis): It is a mere custom that one who gives it to a Shali'ach is exempt. One who gave it to one with a Harsha'ah is exempt according to letter of the law.

Beis Yosef (DH ume'Atah): The Rashba (1024) says that we write a Harsha'ah even on stolen Metaltelim. The Halachah does not follow him.

Shulchan Aruch (ibid.): The Ge'onim enacted to write a Harsha'ah for a loan, whether with a document or with witnesses of a Kinyan, even if the borrower denied it in Beis Din, lest people borrow money and go abroad. They did not enact for a Milveh Al Peh that he denied.

SMA (123:5): They enacted only for what is common. It is uncommon to lend without a document.

Shach (9): They did not enact because it is uncommon, and he denied it.

Rema: In any case, it is no worse than a Shali'ach made in front of witnesses (Siman 122).

Shach (10): This means that if the Nitva gave to the Shali'ach and it was lost, the Nitva is exempt.

Shulchan Aruch (ibid.): Some say that we write for everything, be it a deposit, theft, Milveh Al Peh or with a document, even if it was denied.

SMA (7): This is the opinion of R. Tam and the Rosh. They say that even though one cannot be Makneh them, the custom from the time of the Gemara is that Harsha'ah works for them.

Gra (14): The entire Sugya is unlike Version #1. This opinion holds that also we do not hold like Version #2, for we are not concerned for what looks like Sheker. The first opinion holds like the Rashba (Bava Kama 70a DH u'Mihu), that we are concerned when it will forever look like Sheker.

Gra (15): We learn from Bava Basra 127a that Harsha'ah helps even when they never received the property and cannot be Makneh.