Gemara

(Rav Nachman citing Rav Huna): If a Gazlan (robber) brings a proof, we do not accept it, and we do not establish him to be the owner.

Question: A Mishnah teaches this!

(Mishnah): If Levi bought a field from an extortionist, then bought it from the real owner (Shimon), his purchase is void.

Answer: Rav Nachman teaches that the Halachah is unlike Rav. Rather, the Halachah follows Shmuel.

(Rav): The Mishnah discusses when Shimon told Levi to acquire through Chazakah, but if Shimon wrote a document, Levi acquires.

(Shmuel): Even if he wrote a document, Levi acquires only if Shimon wrote in the document that he accepts Achrayus (to compensate him if the land is legally taken from him).

(Rav Bivi citing Rav Nachman): If a Gazlan brings a proof, we do not establish him to own the land, but he gets back the money he paid.

This is if witnesses testify that the Gazlan paid. If they only saw the owner admit that he was paid, the Gazlan get his money, like Rav Kahana taught;

(Rav Kahana): The owner admitted to the Gazlan out of fear lest the Gazlan be Moser him and his property.

Question: Rav Bivi is unlike Rav Huna, who taught that if Reuven hung Shimon until Shimon agreed to sell his property, the sale is valid.

Answer: Rav Huna argues with Rav Bivi.

Rava: The Halachah is, if Levi hung Shimon until he agreed to sell a field, the sale is valid. If he asked for a specific field, it is invalid, i.e. if Shimon did not count the money, and Shimon had no way to evade Levi. If not, it is valid.

The Halachah is, in all cases the sale is valid, even for a specific field, for it is like Ameimar's law of Kidushin (in which the woman gets no choice):

(Ameimar): If Levi hung Leah until Leah agreed to accept Kidushin from him, the Kidushin is valid.

Rishonim

Rif and Rosh (3:50): Rav Bivi said that if a Gazlan brings a proof, he does not keep the land, but he gets back what he paid, i.e. if witnesses saw him pay. If they only saw the owner admit that he was paid, perhaps he admitted out of fear lest the Gazlan be Moser him and his property. This is when he made a Moda'ah beforehand. If not, even though he was coerced, it is a sale. The Halachah follows Rav Huna, who says that a sale through coercion is valid.

Nimukei Yosef (DH Ein): Even if the Gazlan's document says that the seller received the money, it is invalid. One who comes due to the Gazlan (he bought from him, or received a gift or inheritance) does not keep the land. Shmuel says that Achrayus is a proof, for a Gazlan is particular only about a document, but not about Achrayus. If the seller wrote it, this shows that he sold willingly.

Rambam (Hilchos Gezeilah 9:14): If Ploni stole a field, and after he was Muchzak he bought itr from the owner, and the owner says 'I was forced. I did not sell it willingly', Ploni did not acquire, even if he bought in front of witnesses. We take the field from him and return to him the money he paid.

Rambam (15): This is when the witnesses saw him count out the money. If they testified that the owner sold and admitted that he was paid, and the owner says that he was not paid and admitted only due to fear, Ploni gets nothing. The owner admitted out of fear, since Ploni was established to be a Gazlan on the field.

Rambam (16): The owner need not make a Moda'ah, for the buyer was established to be a Gazlan on this field, so his proof is invalid. A Gazlan is unlike an Ones, i.e. one who coerces or hangs someone until he sells. The Ones does not want to steal, and he did not yet steal anything, therefore, if the seller did not make a Moda'ah, the sale is valid.

Rebuttal (Ra'avad): This is unclear. The Rif disagrees. The Rif's opinion is primary. If Rav Huna distinguishes (an Ones from a Gazlan), why did the Gemara need to say that he disagrees with Rav Bivi? According to the Rambam, Rav Huna agrees with Rav Bivi!

Magid Mishneh: The Rif and Ra'avad hold that if the witnesses did not see money given, the sale is totally Batel. If they saw the money given, the sale is valid. The Sugya equates a coerced sale to a Gazlan who bought. How can the Rambam distinguish them?

Defense (of Rambam - Drishah CM 205:13): Tosfos (47b DH v'Rav) asked why the Gemara said 'Rav Bivi concluded Rav Nachman's teaching to say...' Rav Nachman cited Rav Huna, and we conclude that Rav Huna argueswith Rav Bivi! The Rif, Rosh and Tur hold that Rav Bivi explained that sometimes a Gazlan's proof helps. i.e. to get back his money. The 1

Bach (205:13): The text of the Rif, Rambam and Rosh did not say that Rav Bivi argues. (It is an addition of a Ga'on - Gra 151:4.) The Rif and Rosh hold that Rav Bivi discusses when he made a Moda'ah. The Rambam explains that Rav Bivi discusses a Chamsan (who forcibly pays) with a Moda'ah, and a Gazlan (who does not pay) even without a Moda'ah. It is better to say that the Rif and Rosh agree. This is why they said 'he made a Moda'ah on a sale', and this is why the Tur cites the Rambam without saying that the Rosh disagrees.

Poskim

Shulchan Aruch (CM 151:3): A Gazlan cannot bring a proof. If we know that Ploni stole a field, even if a document says that the owner admitted in front of witnesses that Ploni bought and paid for it, if the owner says 'I did not sell it. I admitted only due to fear', we take the field from Ploni without compensation.

SMA (8): The document says that he was paid, but it does not say that the witnesses saw the money given.

Rema: If the owner wrote Achrayus to the Gazlan, some say that he acquired.

SMA (9): The Tur says that just like a Gazlan on one field can make a Chazakah on another field, for the owner fears to protest only about the field for which the Gazlan risked his life, one does not fear to protest about Achrayus. It is like another field. Therefore, if the owner wrote Achrayus, the Gazlan acquired. If one kills for monetary reasons, he is a Gazlan for everything, and Achrayus does not help for him.

Gra (3): Be'er ha'Golah holds that the Rema says 'some say so', for the Rambam holds that Achrayus does not help for a Gazlan. Really, the Rambam holds that Achrayus never helps. The Nimukei Yosef holds that Achrayus helps for Ones, but not for a Gazlan.

Shulchan Aruch (ibid): If the witnesses say that they saw him count out (and pay) the money, we take the field from him and the owner gives back the money.

Taz: The Tur says that we take the field from him only after the owner gives back the money. This is when the owner did not sell it. If the owner sold it, the buyer takes it immediately from the Gazlan, and the Gazlan requests his money from the owner.

Rema: Some say that if the witnesses saw him pay the money and he did not make a Moda'ah beforehand, the sake is valid. We hold that if Levi hung Shimon until Shimon agreed to sell his property, the sale is valid. This seems primary.

Gra (5): The Rif and Rosh say that Rav Bivi discusses when there was a Moda'ah, and Rav Huna agrees in such a case.

Drishah (6): The Rosh and Tur argue only when the Gazlan did not steal yet, just he made a Chazakah and threatens to steal it if the owner will not sell it. If he already stole it and threatens not to return it if the owner will not sell it, all agree that no Moda'ah is needed.