Gemara

(Beraisa): If Reuven sold a house or field, he cannot testify about it, for there is Achrayus (a guarantee of compensation if it will be taken, therefore, he is partial) If he sold a cow, he can testify about it, for there is no Achrayus.

(Rav Sheshes): In the Reisha, Reuven stole Shimon's field and sold it to Levi, and Yehudah claims that it was his. The Tana teaches that Shimon cannot testify for Levi, for this will help Shimon to get back his field.

Question: We could establish the Beraisa more simply! Reuven stole Shimon's field (and did not sell it), and Yehudah claims that it was his. Shimon cannot testify for Reuven, like we explained above.

Answer: We establish the Reisha regarding a sale for consistency with the Seifa. The case of a cow must discuss a sale, for Reuven keeps it only if the original owner despaired and there was Shinuy (change of Reshus). If it were not sold, it would revert to the owner, so Shimon could not testify about it.

Sukah 30a - Rav Huna (to Hadasim vendors): When you buy Hadasim from Nochrim, do not cut them yourselves. Let the Nochrim cut them.

This is because Stam Nochrim steal land, and land cannot be stolen. If they cut the Hadas, the owner's despair takes effect when it is in the Nochri's hands, and there will be Shinuy Reshus when you get it (so you acquire).

26b (Rava): If one saw a coin fall from Shimon, and he picked it up before Shimon despaired, intending to steal it, even if he returns it after Shimon despaired, this is only a gift. It does not correct his transgressions.

If he picked it up before despair, intending to return it, and after Shimon despaired, he intended to steal it, he transgresses "Hashev Teshivem."

Bava Kama 117b (Mishnah): (If Reuven stole Shimon's field and) the river flooded it, Reuven can say 'your field is in front of you.'

(Beraisa - R. Eliezer): (If Reuven stole Shimon's field and) the river flooded it, he must give him another field;

Chachamim say, he can say 'your field is in front of you.'

They learn what can be stolen from a Kelal u'Frat u'Chlal. "He will deny his neighbor" is a Kelal (general term). "About a deposit" is a Perat (specific term). "Anything that he will swear about" is a Kelal. We include only what is like the Perat, i.e. Metaltelim with intrinsic value, but not land, slaves or documents.

Gitin 39b: R. Yehoshua ben Levi said in the name of Rebbi that if a master despairs of his slave, the only solution for the slave is through a Get.

Since the master despaired, redemption does not apply.

58b (Rav Yosef): The law of extortion does not apply in Bavel.

This is because there is an official with whom complaints may be filed. If one did not complain, this shows that he pardoned the land to the Nochri.

Kesuvos 109b (Mishnah - Admon): If Levi went overseas and returned, and the path to his field was lost, he gets a short path to his field;

Chachamim say, he must pay whatever his neighbor charges for a path, or else he must fly (i.e. he may not walk) to his field.

Rishonim

Rambam (Hilchos Gezeilah 8:14): A thief can never acquire land. It is always in the Reshus of the owner, even if it was sold 1000 times and the owner despaired, it returns to him for free. Whomever it was taken from demands his money back from the one who sold to him, and the seller demands his money back from the one who sold to him, until the first buyer claims from the thief.

Rosh (3:41): Despair applies to land. If an extortionist stole land and planted Kilayim and the owner got it back, he uproots the Kilayim. The Yerushalmi says that if people forgot who is the true owner but he did not despair, it is forbidden mid'Oraisa. If people forgot the owner and he despaired, it is forbidden mid'Rabanan. Even though land cannot be stolen, despair applies.

Hagahos Maimoniyos (4): The Yerushalmi holds that despair takes effect on land. The Bavli disagrees; it is primary.

Bach (CM 371:1): Hagahos Maimoniyos refers to Chachamim in the Beraisa (Bava Kama 117b). The Stam Mishnah is like them.

Shach (2): In Bava Basra, Tosfos and the Rosh say that despair takes effect on land. The Tur rules like the Rambam, Rashbam, Rashi and others who disagree. Chachamim in Bava Kama say only that land cannot be stolen. The Yerushalmi also says so! Rather, the Sugyos in Bava Basra and Sukah show that despair does not take effect on land. Tosfos explained differently, but his Perush is not primary.

Rashbam (44a DH u'Farkinan): The Beraisa discusses despair with Shinuy Reshus, for in this case we distinguish land from Metaltelim. The buyer acquires the Metaltelim, but not land, for it cannot be stolen.

Tosfos (44a DH Davka): The Yerushalmi says that despair helps for lamd, even though it cannot be stolen! We find that despair helps for slaves (Gitin 39b), even though they are equated to land! Gitin 58b also proves that despair helps for land. Rather, the Reisha discusses when there was no despair (from land, and the Seifa discusses when there was despair from Metaltelim). We could have distinguished between before and after despair or Metaltelim. The Beraisa discusses land, for normally one does not despair from land. Even if the thief does not heed Beis Din, perhaps his son will, and the owner will get it from his son.

Rashi (Sukah 30b DH v'Karka): Land is not acquired to a thief through any despair, like we learned in Bava Kama. Peros can be acquired through despair after they are detached.

Milchamos Hash-m (Bava Metzia 14b): If one took an Aveidah with intent to steal it, it is as if it is on the ground, and it is acquired through despair, like an Aveidah. If he took it with intent to return it, he is like a Shomer of the owner. It is as if it is in the owners' Reshus, therefore, despair does not help.

Poskim

Shulchan Aruch (CM 371:1): Land can never be stolen. It is always in the owners' Reshus. Even if it was sold 1000 times and the owner despaired, it returns to him for free. Whomever it was taken from demands his money back from the one who sold to him, and the seller demands his money back from the one who sold to him, until the first buyer claims from the thief.

Beis Yosef (DH u'Mah she'Omar ba'Meh): Admission of land is not Heilach (returning what one admits that he owes, to exempt from the oath of partial admission) in a case when he dug pits (for he must pay for the loss - Bava Metzia 5a).

SMA (1): Land always belongs to the owner. There is no enactment that a buyer gets what he paid for land when the owner takes it back, for a buyer can easily find out whose it is.

Nesivos ha'Mishpat (Bi'urim 1): The Torah taught that land cannot be stolen, but why shouldn't despair work? We learn about despair from an Aveidah. Why shouldn't it apply to land, slaves and documents? Kapos Temarim (Sukah 30b DH ul'Inyan) proves from Gitin 58b that despair applies to land. (Note: he cites the Gemara to say 'he despaired'. Our text says 'he pardoned.') The Terumas ha'Deshen (310) says that Chachamim hold that if the path to one's field was lost, he does not get any path because he despaired. Also, why does the Yerushalmi say that despair acquires alone? Even Metaltelim require Shinuy Reshus with despair! Milchamos Hash-m explains that despair helps only for something not in his Reshus. If one's land was stolen and he did not despair, he cannot be Makdish it for it is not in his Reshus (Bava Metzia 7a). If one's land was stolen he can be Makdish it, for it is in his Reshus. Therefore, despair does not apply to land, i.e. when he could get it through Beis Din. The Halachah is that one loses the path to his field when different neighbors surrounded him, and he cannot prove who took his path. The Yerushalmi says that despair helps when the owner was forgotten and he cannot get it back. If there was despair but no Shinuy Reshus, there is a Mitzvah to return it. Since land is not stolen, there is no Mitzvah to return it, therefore, despair helps without Shinuy Reshus.