Gemara

(Shmuel): Reuven can testify for his partner Shimon.

Question: Why is this permitted? The testimony affects Reuven!

Answer: The case is, Reuven wrote to Shimon 'I have no claim to this field (and did a Kinyan Chalipin and accepted Achrayus if his creditor will collect it).

Question (Beraisa): If the Sefer Torah of a city was stolen, people of the city cannot judge or testify about it (since it affects them. We do not say that one can be Mesalek (forfeit his share in it) and judge the case!)

Answer: A Sefer Torah is different, for everyone must hear it read.

Kidushin 56a (Mishnah): One may not buy a Tamei animal, slaves or land with Ma'os Ma'aser, even in Yerushalayim. If he bought, he must set aside money equal to what he paid and consume it like Ma'aser.

Question: A Beraisa says that the sale is undone!

Answer (Shmuel): The Mishnah discusses when the seller fled.

Bava Kama 68a (Rava): If Reuven stole an animal and sold it, and (before despair) then Shimon stole it, Shimon pays only principal.

68b (Mishnah #1): Tzenu'im (pious people, who were concerned lest people taking Shemitah produce mistakenly take Revai fruits of a tree in its fourth year, which must be eaten in Yerushalayim or redeemed)) would say 'whatever (Revai) was gathered from my field is redeemed on these coins.'

69a (R. Yochanan): Tzenu'im and R. Dosa hold like each other:

(Beraisa - R. Yehudah): In the morning, the owner says 'what the poor will gather today (mistakenly, thinking that it is Leket) is (now) Hefker';

R. Dosa says, at evening he says 'what the poor gathered is Hefker'.

R. Yochanan taught that 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. He holds like a different Stam Mishnah.

(Mishnah #2): One who steals from a thief does not pay Kefel. If he then slaughters or sells it, he does not pay four and five.

He doesn't pay the thief, for it is not the thief's. He doesn't pay the owner, for it was not in his Reshus.

Rishonim

Rif and Rosh (Bava Basra 23a and 3:39,40): Reuven can testify for his partner if he wrote to him 'I have no claim to this field', and did Chalipin and accepted Achrayus. If the Sefer Torah of a city was stolen, people of the city cannot judge or testify about it, for everyone must hear it read. If it was something from which they can be Mesalek, they can do so and judge the case. This is the Halachah.

The Rif (Bava Kama 27a) brings R. Yochanan's law.

Rambam (Hilchos Erchin 6:22): One cannot be Makdish something not in his Reshus. If Ploni had a deposit with Shimon who denied having it, Ploni cannot be Makdish it. If Shimon did not deny it, it is in Ploni Reshus whenever it is.

Question: One who bought land with Ma'aser coins must eat in Yerushalayim corresponding to them. How can he redeem coins out of his Reshus?

Answer #1 (Tosfos Bava Kama 68b DH Hu): The seller is like a thief. If he were here, he would have to return it. The Mishnah is like Tzenu'im, who say that one can be Makdish (or redeem) what is in a thief's hands. Alternatively, the buyer does not redeem the money. It is a mere fine to eat equal to the money he paid.

Answer #2 (Ritva Kidushin 56a DH Maskif): The seller did not intend to steal the coins. If he were here, he would obey Beis Din! He just does not know. Therefore, the coins are like a deposit with him, and the buyer can redeem them.

Ketzos ha'Choshen (354:4): Tosfos' opinion is primary. R. Yochanan taught that Tzenu'im and R. Dosa hold like each other: Aniyim err about what is Leket, yet what they took is considered not in the owner's Reshus. It is like a deposit only when the owner can get it back whenever he wants. Also, if one stole an animal and sold it, and then (before despair) Shimon stole it, Shimon pays only principal. This is because it is not considered to be in the owner's Reshus, even though the buyer erred and would return it if he knew.

Question (Tosfos ibid.): How can we learn about Hekdesh from Chilul? Chilul is advantageous for the own with the Peros, and Zachin l'Adam she'Lo Befanav!

Answer (Tosfos Kidushin 56a DH Maskif): One who took what is not really Leket does not own it, so he cannot redeem it. Zechiyah works due to Shelichus; one cannot do for someone what he cannot do himself.

Poskim

Shulchan Aruch (CM 354:6): If Shimon openly or covertly stole Metaltelim from Levi, and Levi did not despair, neither of them can make it Hekdesh. This is even if witnesses saw the theft, and Levi can recover it in Beis Din.

Ketzos ha'Choshen (123:1 DH mi'Dina): One version (70a) says that one cannot write a power of attorney for Metaltelim, due to R. Yochanan's law. Rashi says that we discuss a Shomer. I.e., the Shomer has a claim against the owner and refuses to return the deposit before Beis Din rules about his claim. Therefore, the owner cannot be Makneh it.

Rashba (70 DH veha'Ramban): Hekdesh is a Gezeras ha'Kasuv, but one can be Makneh something even if it is not in his Reshus. According to the opinion that a loan is not given to be spent, one can be Mekadesh a woman with it, even though it is not in his Reshus!

Question (Ketzos ha'Choshen 123:1 DH Ayen): We learn from Hekdesh that one cannot be Makneh something not in his Reshus! One cannot even be Mafkir it, for R. Yochanan said that Tzenu'im and R. Dosa hold like each other! If a loan is not given to be spent, one can retract, so it is like a deposit, therefore it can be Mekadesh. Even if it were considered theft, one can be Makneh it to the thief. If one stole from a woman and was Mekadesh her with it, she is Mekudeshes, for she is Makneh it to him ( Kidushin 13a).

R. Akiva Eiger (YD 258:7): If Shimon wants to return the theft, Tosfos (Bava Metzi'a 6a DH Hikdisho) and the Ba'al ha'Ma'or (Bava Kama 18a) say that Ploni can be Makdish it now. Milchamos Hash-m disagrees. The Ramah (in Shitah Mekubetzes Bava Kama 33a) says that if he was Makdish it, it takes effect after he receives it through Beis Din.

Gilyon Maharsha: R. Akiva Eiger asked, how can we say (Bava Basra 43a) that one can be Mesalek and then judge the case? One cannot be Makneh what is not in his Reshus! I answered according to the Ramah (the pardon helps after it is returned). The Rashba says that one can pardon not only to the thief, rather, even to his partner. Perhaps this is like he holds, that one can be Makneh something even if it is not in his Reshus. The Rashbam says that the thief claims that he bought it. Since he agrees to return it, just he wants to be paid for it, it is considered in the owners' Reshus. Some Rishonim do not require witnesses to be Kosher from the beginning for monetary cases. Why didn't the Gemara ask that witnesses be Mesalek and testify? The Rashbam mentions 'testimony that he stole.' I.e. when he refuses to return the object, Siluk does not help.