Gemara

(Beraisa): Chachamim later decreed that also a Gazlan (robber) and Chamsan (one who forces another to sell) are disqualified from testimony.

Originally, they did not disqualify Chamsanim, for they pay, just they coerce the owner to sell (so the sale is valid);

Later, they disqualified them, because they saw that they take and give money, even if the owners do not agree to sell.

Bava Kama 62a - Question (Rav Ada brei d'Rav Avya): What is the difference between a Gazlan and a Chamsan?

Answer (Rav Ashi): A Chamsan pays for what he takes. A Gazlan does not.

Question (Rav Ada): One who pays money is not called a Chamsan!

(Rav Huna): If Levi hung Shimon until Shimon agreed to sell his field, the sale is valid.

Answer (Rav Ashi): The sale is valid if Shimon said 'I want to sell it'. If not, it is invalid (and Levi is a Chamsan).

Bava Basra 48a: Rav Huna holds that the sale is valid because amidst the coercion, he has Gemiras Da'as (decides absolutely) 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 resolves to sell!

Answer: Indeed, if Levi paid Shimon, the sale is valid. Rav taught that the Mishnah discusses when Shimon told Levi to acquire through Chazakah. If Shimon wrote a document (we assume that he was paid, so) Levi acquires.

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): We conclude like Rav Huna that in every case the sale is valid, even if he asked for a specific field, even if money was not given, and only a document was written. The seller must accept the money afterwards.

Rambam (Hilchos Mechirah 10:1): If Shimon was coerced until he agreed to sell and he took the money of 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.

Rebuttal (Ra'avad): The Rambam misunderstood the Rif. The Rif meant that even though he did not give the money at the time he hung him or of the sale, since he later paid, the sale is valid. However, he must pay in front of witnesses, or else the seller can say that he was never paid.

Magid Mishnah: The Ra'avad understands that the Rambam discusses even when the seller says that he was not paid. He assumes that in a normal sale, the buyer is believed immediately after to say that he paid. I do not know a source for this. The Ba'al ha'Itur says in the name of R. Efrayim that within three years the seller is believed to say that he was not paid. The Rambam understands that the discussion about whether or not he appeased the seller refers to in front of witnesses. The conclusion is that even if it was not in front of witnesses the sale is valid, as long as the seller admits that he was paid. We do not say that the admission was coerced, unless there was a Moda'ah. Perhaps the Ra'avad requires that the witnesses saw the seller count the money, for if not, he can say that he was forced to admit.

Beis Yosef (CM 205 DH uv'ha'Nizakin): It seems that the Rif holds that he acquires through the document alone, even without money, because they intended that the buyer will pay. The Ra'avad understands that the Rif holds that he acquires only when he later accepted the money. If so, the Rambam is like the Rif. What forced the Ra'avad to say so?

Lechem Mishneh: We learn from Bava Kama (62a) that the sale is valid only if the seller said 'I want'. If not, the buyer is a Chamsan. Why did the Rambam omit this? The Magid Mishnah (Hilchos Ishus 4:1) asks why the Ra'avad says that if a man was forced to be Mekadesh it is valid only if he said 'I want', but did not make this condition regarding a sale. The Ra'avad made this condition, in Hilchos Gezelah (1:9). The Maharik says that regarding a sale perhaps it suffices if he received the money in silence. We rely on this until we find a better answer (for the Rambam).

Rosh (Bava Kama 6:17): A Chamsan is one who pays for what he takes, but the seller did not say 'I want'. Mid'Rabanan he is Pasul (for testimony). In the Torah "Chamsan" refers to a proper Gazlan, but Chachamim use the word differently.

Tosfos (Bava Kama 62a DH Mah): The Gemara asked what is the difference between a Gazlan mid'Rabanan and a Chamsan mid'Rabanan. (Chachamim disqualified both from testimony - Sanhedrin 25b.)

Rebuttal (Gra CM 205:1): If so, what was the question from Rav Huna?! (Even though the sale is valid, Chachamim disqualified him for transgressing "Lo Sachmod"!)

Poskim

Shulchan Aruch (CM 34:13): A Chamsan, i.e. one who takes land or Metaltelim Bal Korcho (against the will) of the owner, even though he pays the money, is disqualified (from testimony) mid'Rabanan.

Shulchan Aruch (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.

Beis Yosef (DH Mi and DH u'Mah she'Chosav Rabeinu): The Rashbam says that if Lo Artzei (he did not count the money), this shows that he was forced to sell. Also the Tur explains like this, and rules like Rav Huna, that the sale is valid in every case, but only if he said 'I want.' Perhaps acceptance of the money in silence is like saying 'I want.'

Gra (1): If the buyer forcibly gave the money and took the item, this is not a sale. The seller must say 'I want.' Taking the money is like saying 'I want.' (Note: R. Yisrael mi'Shokolov (introduction to Bi'ur ha'Gra in Shulchan Aruch ha'Shalem) says that the Likutim in Bi'ur ha'Gra were written later. If so, the Gra's words below show that he retracted from what he wrote here.)

Gra (1, Likut): The Shulchan Aruch does not mention that the seller said 'I want.' The Sugya in Bava Kama asked what is the difference between a Gazlan and Chamsan. It holds that a Chamsan is (a thief) mid'Oraisa (for he does not acquire, unless the seller consented). The Shulchan Aruch rules like Sanhedrin 25b, which disqualifies a Chamsan only mid'Rabanan (for it holds that he acquires in any case). This is why the Rambam (Hilchos Gezeilah 1:9) did not mention a Chamsan.

SMA (1): The Rif, Rambam and Shulchan Aruch connote that even if we do not know about the Ones, and we see that the seller does not sell willingly, we say that since he received money, the sale is valid. The Rosh and Tur hold that if he sold unwillingly it is invalid, unless there was Ones, for then amidst the Ones he resolves to sell.

SMA (3): Hanging is a great Ones. All the more so, if there was a small Ones he cannot annul the sale due to Ones.

Shulchan Aruch (359:9): One may not force another to sell and pay the money, but he is not a Gazlan who is disqualified (from testimony) mid'Oraisa. He is disqualified mid'Rabanan.