Gemara

(Beraisa): If Levi hired David to work for himself, and set him to work on what is really Reuven's property, Levi pays David, and can demand from Reuven the benefit Reuven received.

Gitin 58b (Beraisa): The law of Sikrikon (that Levi can buy the land from the Nochri who took from Reuven and give Reuven a quarter of the price) does not apply to a Nochri who took Reuven's land for payment of a debt, or through Anperos (extortion (when there was no concern lest he kill Shimon);

Regarding Anperos, it must remain with the Nochri for 12 months. (If not, the land belongs to Reuven.)

Question: We said that the law of Sikrikon does not apply to Anperos!

Answer: Rather, the Beraisa means that regarding a Sikrikon, the land must stay with the Nochri for 12 months.

(Rav Yosef): There is no (law of) Anperos in Bavel.

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

Rishonim

Rif and Rosh (Gitin 27a and 5:17): There is no law of extortion in Bavel, for there is an official. If Reuven did not complain, he pardoned the land to the Nochri.

Rambam (Hilchos Gezeilah 10:1): If a powerful Nochri forcibly took Reuven's field because Reuven owed him money, or Reuven caused damage or a loss to him, and after he seized the field he sold it to Levi, Reuven cannot take it from Levi.

Rambam (2): This is when Yisrael witnesses testify that the Nochri's claim was true, or Reuven admits to it. Or, it is even without witnesses or an admission, if there was a king or official who could have forced the Nochri in judgment, and Reuven didn't claim. Levi can say 'if he is a thief, why didn't you claim from him?'

Rosh (Bava Kama 6:7): R. Gershom ruled that if a Nochri forcibly took Reuven's house and Levi bought it from the Nochri, he need not return it for free. Rather, Reuven pays the amount Levi benefited him. Beis Din estimates how much he would pay to get his land back. This is like one (Levi) who hired a worker and told him to work in Reuven's field. Levi pays the worker his full wage, and Reuven pays Levi the benefit he received (Bava Metzia 118a). In Gitin, Rashi explained that if the Nochri took the field for a debt, or through extortion, Levi returns it for free. Reuven never intended to give it to the Nochri. Even if he had it for 12 months, Reuven merely had no opportunity to force him to return it. The Ri holds like R. Gershom. This is unlike Mavri'ach Ari (one who prevented potential damage, and is not compensated for what he spent). Even though he intended, since it came to the Nochri's Reshus, Reuven is unlikely to get it back for free. Levi is like one who saved from a lion. He receives the benefit he gave. He does not get everything he spent, like a shepherd who hired (others or) sticks to fight off a lion. There, he intended to help the owner of the flock. Here, he bought it for himself, so he gets only Reuven's benefit.

Question (Ketzos ha'Choshen 236:1): Teshuvas ha'Rosh (95:2) says that R. Gershom obligates giving all that Levi paid. This requires investigation. We follow Piskei ha'Rosh against his Teshuvah. If Levi intended to return the field to Reuven, Piskei ha'Rosh agrees that he gets all that he paid.

Poskim

Shulchan Aruch (CM 236:7): If a powerful Nochri forcibly took Reuven's field because Reuven owed him money, or for damage or a loss to him, and then that is Nochri sold it to Levi, Reuven cannot take it from Levi. This is when Reuven admits that the Nochri's claim was true, or there was an official who could have forced the Nochri in judgment, and Reuven didn't claim. This is even if Reuven does not admit to the Nochri's claim and there are no witnesses. Levi can say 'if he is a thief, why didn't you claim from him?'

SMA (15 and Shach 3): This is like the Rambam. The Tur holds like the Rosh citing R. Gershom, that Reuven gets it back. He owed only money to the Nochri, so if he pays this to the buyer, he gets back his land.

Shulchan Aruch (8): If a Nochri came with pretexts and took Reuven's land, but he could not have killed him, and the Nochri sold or gave it to Levi, Levi must return it to Reuven, even if he had it many years. We estimate how much Reuven would have wanted to pay the Nochri to get it back; he pays this to Levi.

Beis Yosef (DH Ba): R. Gershom says that Levi must return it to Reuven even if he had it many years. The Mordechai says that Rashi disagrees, but many hold like R. Gershom, and we rule like them.

Drishah (4): The Beraisa discusses a Nochri who took land for a debt, or through mere extortion. Rashi explains that he had no proof that he was owed, like the case of Anperos (extortion). The Rambam explains that the Nochri has witnesses about the debt, or the Yisrael admits. Anperos is similar, i.e. he has witnesses that the Yisrael caused a loss or damage to him. The Rambam must explain 'there is no law of extortion in Bavel', i.e. Reuven does not get his land back even if no witnesses support the Nochri's claim. Perhaps Rashi and the Rambam argue only about how to explain the Beraisa, but l'Halachah they agree.

SMA (19): Reuven pays Levi only what Reuven would have needed to pay.

Shach (4): If Levi intended to return it to Reuven, Reuven must pay him as much as he paid, up to its value.

Shulchan Aruch (ibid.): If Levi spent to buy it in order to get it from the Nochri, Reuven returns to him up to the house's value. He need not pay him more than he paid, e.g. the Nochri liked Levi and sold to him cheaply, and Reuven could not have bought it for so little. If it was a gift, Reuven gets it for free.

Beis Yosef (DH v'Chosav Adoni): The Rosh (95:2) says that Levi does not get more than he paid. He cannot do business with another's property!

Rema: This applies only to land, for people do not despair from it. People despair from Metaltelim, so Levi need not return to Reuven, except for Seforim. People do not despair from Seforim, for they are sold only to Yisre'elim.

Shulchan Aruch (CM 356:2): If Levi bought from a known thief, there is no enactment. Reuven (the owner) takes his object back, and Levi claims from the thief.

Rema: Some say that if Levi did not know that this item was stolen, there is an enactment (Reuven pays him what he paid, and Reuven claims from the thief). If Levi says 'I intended to help Reuven get back his item', he is believed. All agree that Reuven must pay him what he paid, even if Levi bought from a known thief.

Tosfos (114b DH ha'Makir): Even without an enactment, Levi can get what he paid, for he can say that he intended to save it (return it to the owner)! We must say that the case is that Reuven could have claimed from people who lodged in his house (surely, they stole it), even if no one bought it.

Ketzos ha'Choshen (3): We learn from Tosfos that if he did not intend to save, he is paid only due to the enactment. When there is no enactment, e.g. the thief is notorious, he is not paid anything. However, it seems that even then he gets what he benefited Reuven, for Reuven could not have gotten back his item otherwise. This is unlike Mavri'ach Ari, for he saves him from loss. This is even if Levi intended to buy for himself, and not to save. However, when he intended to save he gets all his expenses. When he intended to buy for himself, he gets only what he benefited Reuven.

Nesivos ha'Mishpat (Urim, 236:7): Here, when Levi bought from a known thief, Reuven takes his object back for free, unless Levi intended to return it to the owner. In Siman 236, Levi bought from a Nochri, and gets what he benefited Reuven, regardless of his intent, even though a Nochri is like a known thief! The Ketzos ha'Choshen says that even here, Levi receives what he benefited Reuven, regardless of his intent. I say that also here, he is paid only if he intended to return it, for then he is like one who improved another's field without permission. If Levi bought it for himself, Reuven does not pay him, since Reuven does not benefit from this (only his property benefits). The Rema relied on what he wrote in Siman 356 that he is paid only if he intended to help Reuven. The Shulchan Aruch (236:8) alluded to this by saying 'he spent money to buy it in order to get it from the Nochri.'