Gemara

47b (Rav Huna): If Levi hung Shimon (or afflicted him in another way) until Shimon agreed to sell his property, the sale is valid.

Amidst the coercion, he has Gemiras Da'as (resolves) to sell.

Question (Rav Hamnuna - Mishnah): If Levi bought a field from an extortionist, then bought it from the real owner (Shimon), his purchase is void. (We should say that amidst the coercion, he has Gemiras Da'as to sell!)

Answer: Indeed, if Levi paid Shimon, the sale is valid.

Question: This is like Rav, who says that the Mishnah discusses a sale through Chazakah, but If Shimon wrote a document (we assume that he was paid, so) Levi acquires. According to Shmuel, how can we answer?

(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).

Answer: Shmuel agrees that if Levi paid Shimon, the sale is valid.

Question: According to Rav Bivi, how can we answer?

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.

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 (Gitin 27a and 5:16): The Halachah follows Shmuel against Rav. We conclude like Rav Huna that in every case the sale is valid, even if he asked for a specific field, and no money was given, and only a document was written. The seller must accept the money afterwards. This is like Rav, who does not require Achrayus! This is not difficult. Shmuel admits that he acquires without Achrayus when money was or will be given, e.g. in a forced sale. Shmuel requires Achrayus only in cases like an extortionist, who will not pay.

Rif and Rosh (3:50): Rav Bivi said that if a Gazlan brings a proof, he gets back what he paid, i.e. if a Moda'ah was made. If not, even though he was coerced, it is a sale. The Halachah follows Rav Huna, who validates a coerced sale. This is logical. Amidst the coercion, he has Gemiras Da'as to sell. Rava rules that this is when Levi asked to buy a field. If he asked for a specific field, and Shimon did not count the money, and he had no way to evade Levi, it is invalid. If not, it is valid. The Halachah is, in every case it is valid, even regarding 'this field', for a woman is like a case of this field, and Ameimar said that forced Kidushin is valid.

Nimukei Yosef (DH Ein): If one was hung and gave a gift, it is invalid. The Ritva says that since it depends on the seller's Da'as, we cannot learn entirely from a woman, and say that just like if she was coerced and accepted a Perutah for Kidushin, it is valid, also if a seller received a Perutah for his field. Any woman can be Mekudeshes for a Perutah. Also, a woman prefers to be married. If a woman made a Shali'ach to accept Kidushin and he received a Perutah, she is Mekudeshes. If one made a Shali'ach to sell a field and he erred (received less than its value), the owner can retract. It seems that the Ran agrees, even though some totally equate the sale of a field to Kidushin.

Rambam (Hilchos Mechirah 10:1): If Shimon was coerced until he agreed to sell and he took the money for the goods, even if he was hung until he sold, his sale is valid. This applies to Metaltelim and land, for amidst the coercion he had Gemiras Da'as to sell. This is even if he did not take the money in front of witnesses. Therefore, if he made a Moda'ah before he sold, and told two witnesses 'I am selling this item or field to Ploni because I am forced', the sale is void. Even if Ploni made a Chazakah (used what he bought) for many years, Shimon takes it back and returns the money.

Kesef Mishneh: The Rambam (Hilchos Gezeilah 9:15) holds that if someone stole a field and later bought it from the owner, if witnesses did not see the owner count the money he is believed to say that he admitted out of fear, even without a Moda'ah. We give the field to him; he need not pay. Here, we do not know that the buyer once stole the field, therefore the owner is not believed to say that he admitted out of fear unless he made a Moda'ah.

Mishneh l'Melech: Perhaps the Rambam says 'he took the money of the goods' to disagree with those who do not require that he paid the full value. It is better to say that the Rambam says this to disagree with the Rif, who does not require that he pay at the time of the sale.

Poskim

Shulchan Aruch (CM 205:1): If Shimon was coerced until he agreed to sell and he took the money, even if he was hung until he sold, his sale is valid for Metaltelim or land, for amidst the coercion he had Gemiras Da'as to sell. This is even if he did not take the money in front of witnesses.

SMA (4): This opinion relies on admission that he was paid, for he did not make a Moda'ah about this. The Rema brings the other opinion, that the Moda'ah on the sale proves that he was forced also to admit.

Gra (4): This is like the Rif, who says that Lo Artzei means that he did not give the money. We cannot say that he did not give at all, for the Halachah follows Shmuel! Rather, he did not give in front of witnesses.

Rema: Some argue and say that the witnesses must see the money given, but admission does not suffice.

Gra (5): Rav Bivi said that they must see the money given. There is no source to say that Rav Huna argues about this.

Rema: Some say that he must give money, but a document (saying that he owes) does not acquire.

Gra (6): The Halachah follows Shmuel in monetary laws. Shmuel admits that he acquires when money was given.

Rema: Some disagree.

Gra (7): The Rif explains that Shmuel admits when the seller expected to get paid, even if he was not paid at the time of the sale.

Shulchan Aruch (ibid): Therefore, if he made a Moda'ah before he sold and told two witnesses 'I am selling this item or field to Ploni because I am forced', the sale is void. Even if Ploni made a Chazakah for many years, Shimon takes it back and returns the money. The witnesses must know that he sells due to the Ones and he is truly forced. They may not rely on what he says. Any Moda'ah in which the witnesses did not write 'we know that he is Anus' is invalid.

SMA (7): This connotes that the witnesses must know that he did not resolve to sell amidst the Ones. I say that they need not know this. Perhaps also the Shulchan Aruch only requires that they know of the Ones.

Shulchan Aruch (2): This refers to a sale. For a gift or pardon, the witnesses need not know his Ones.

SMA (8): Only regarding a sale, we say that since he received money, he resolved to sell.

Shulchan Aruch (8): The sale is valid if there was no Moda'ah regarding a Chamsan, who forces the owner to sell unwillingly. If one stole a field and bought it afterwards, no Moda'ah is needed.

Beis Yosef (DH ba'Meh): The Rambam holds that the Gemara said that Rav Huna disagrees with Rav Bivi, i.e. according to Rav Hamnuna. Really, they do not argue. Alternatively, Rav Huna disagrees with Rav Bivi, but Rav Bivi holds like Rav Huna. The Halachah follows Rav Bivi, for he cited Rav Nachman, and the Halachah follows Rav Nachman in monetary laws.