Gemara

(Rav Avdimi bar Chama): "Va'Yisyatzvu b'Sachtis ha'Har" teaches that Hash-m bent Har Sinai over Yisrael like a vat. He told them "if you accept the Torah, good. If not, you will be buried there."

(Rav Acha bar Yakov): This is a great Moda'ah for Kabalas ha'Torah! (It shows that the acceptance was forced, and therefore invalid.)

Bava Basra 47b (Rav Huna): If Levi hung (coerced) Shimon until Shimon agreed to sell his property, the sale is valid.

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

The Halachah is, whether he asked for any field or a specific field, the sale is valid, for it is like 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.

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

(Shmuel): Even if Shimon wrote a document, Reuven acquires only if Shimon wrote in the document that he accepts Acharayus.

48a: Shmuel agrees that if Reuven paid Shimon, the sale is valid.

Rishonim

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.

Rambam (Hilchos Ishus 4:1): If a man was Mekadesh a woman against her will, she is not Mekudeshes. However, if a man was forced to be Mekadesh, she is Mekudeshes.

Magid Mishnah: The Rambam holds that Chachamim uprooted Kidushin when she was forced, for she cannot choose to leave the marriage. They did not uproot Kidushin when he was forced, for he can divorce her if he wants. Ba'al ha'Itur holds that in both cases, Chachamim uprooted it.

Ra'avad: If he was forced, it is valid only if he said that he wants.

Rebuttal (Magid Mishnah): It is valid even if he did not say that he wants, just like a sale. The Ra'avad did not make this condition regarding a sale.

Rashba (Kidushin 2b DH Tana): If a man was coerced to be Mekadesh, the Rambam says that she is Mekudeshes. Ba'al ha'Itur disagrees, for this is like one coerced to buy. The Rambam is correct. If amidst coercion a person has Gemiras Da'as to sell, all the more so coercion gives Gemiras Da'as to buy!

Mordechai (Gitin 395): What a person vowed or swore under coercion is invalid. He can permit it himself, without a Chacham, even if it is possible to fulfill it. Rava taught that there was a Moda'ah against Kabalas ha'Torah, even though we could fulfill the Torah, and Hash-m wanted. All the more so, if people coerced someone to vow, against Hash-m's will, it is void even if he could fulfill it.

Poskim

Rema (CM 205:12): Only regarding a seller, we say that if he was coerced, amidst the coercion he resolved to sell. If one was coerced to buy, it is not a Kinyan.

Beis Yosef (DH Kasav): Ba'al ha'Itur writes that if one was coerced to sell, we say that amidst the coercion he resolved to sell. If one was coerced to buy, we do not say that amidst the coercion he resolved to buy. This is like we say, that Shmuel agrees about one who gave money.

Gra (32): I wrote in EH 41 (brought below) that all the Poskim disagree.

Ketzos ha'Choshen (9:1): What is done through coercion is Ones, and unwillingly. Even though we coerce someone until he agrees to offer a Korban, and this is not considered b'Al Korcho (Bava Basra 48a), Tosfos (DH Ileima) explains that what Halachah obligates someone to do is considered like a sale. He does not do it for naught. (Therefore, it is valid.) This refers to one forced to sell. If one is forced to be Zocheh (acquire), this is like being forced to buy. It is invalid, like the Rema brings from Ba'al ha'Itur. This is why we do not take a security from one obligated to bring a Chatas or Asham, since they are for Kaparah. We should force him to say that he wants! Kaparah requires desire. This is like buying, for which coercion does not help. We force for an Olah, for to be Makdish an Olah is like selling.

Nesivos ha'Mishpat (Bi'urim 18): If Ploni coerced Reuven to sell, even if he forced him to accept Metaltelim for payment, just like we say that amidst the coercion he resolved to sell, here also we say that amidst the coercion and the Shavah Kesef he resolved to sell, for he can sell the Shavah Kesef for money. It is as if he received money. Do not say that Reuven did not acquire the object he was given for Chalipin, for one who is coerced to buy did not acquire, and therefore Ploni does not acquire. This is wrong. Anyone who sells for Shavah Kesef, which is given in place of money, he does not acquire the Shavah Kesef, just the buyer is obligated (Bava Metzi'a 45b). Gold obligates [one who bought it for silver, to pay] silver. Since Reuven does not acquire the item itself, we cannot apply to him the law of one coerced to buy. Rather, he was coerced to sell, and he resolved to sell.

Nesivos ha'Mishpat (ibid., DH u'Lefi): Based on this, if Ploni designated an item and forced Reuven and said 'sell to me for this', this is Chalipin (Rema 203:3). Therefore, just like Reuven does not acquire, for he was forced to buy, also Ploni does not acquire. Also, even if Ploni merely gave a security for the money, he acquired. Many Poskim hold that becoming obligated to pay suffices to acquire (Rema, Sa'if 1). It seems that all agree that getting a security is like receiving money. The Mechaber (EH 42:1) brings an opinion that if one was coerced to be Mekadesh, she is Mekudeshes, for this is like one who was coerced to buy. Chelkas Mechokek and the Beis Shmuel were astounded why this opinion was not brought here in Choshen Mishpat. It seems that it is a mere stringency of Kidushin. If Ploni coerced Reuven to buy, Reuven can be Mevatel the purchase, but Ploni cannot. This is like 200:11 (regarding one who took an item to test it); the seller cannot retract. Similarly, here it is as if he stipulated that the Kinyan depends on the buyer's will.

Chasam Sofer (Teshuvah 3:112): A man was Mekadesh a woman, and does not want to make Nisu'in. We could force him through the mayor. However, EH 42:1 has a Safek about a man forced to be Mekadesh. The Chelkas Mechokek and Beis Shmuel say that mid'Oraisa it is Kidushin, unlike the Rema in CM 205:12. The only question is whether Chachamim uprooted the Kidushin. I say that we may force him. However, I supported the Rema from Shabbos 88a, which says that there was a great Moda'ah on Kabalas ha'Torah. There, there was mortal coercion. What was the Moda'ah?! The Mishneh l'Melech challenged the Mordechai (Gitin 394), who says that through mortal coercion, one resolves to give. It is as if he sells in exchange for his life. The Mordechai himself learns from the Moda'ah on Kabalas ha'Torah to coerced oaths! This supports the Rema. If one is mortally coerced to sell, even if he does not receive money, he resolves to give, for every seller is coerced due to need of money. All the more so, he resolves to save his life! This refers to a seller. However, one buys only willingly. Perhaps even mortal coercion does not help. Therefore, there was a Moda'ah on Kabalas ha'Torah. Hash-m is called the seller, and it is "Lekach" (a purchase) for Yisrael (Berachos 5a).

Pischei Teshuvah (11): The Chelkas Mechokek and Beis Shmuel proved from the Rambam and Magid Mishneh that coercion helps for buying just like for selling. The Rashba says that all the more so it helps for buying.

Shulchan Aruch (EH 42:1): If a man was Mekadesh a woman against her will, she is not Mekudeshes. However, if a man was forced to be Mekadesh she is Mekudeshes. Some say that she is not. Therefore, it is a Safek.

Chelkas Mechokek (1): The Beis Yosef, citing Ba'al ha'Itur, and Rema disqualify a forced purchase. This is difficult. The Magid Mishnah equates a man forced to be Mekadesh with a woman forced to accept. Just like a forced sale is valid, also a forced purchase is valid.

Question (Avnei Milu'im 1): We force a Me'anes (rapist) to marry her, if she and her father want. The Ba'al ha'Itur disqualifies a forced Kidushin!

Avnei Milu'im (1): The Rashba holds like the Rambam, unlike Ba'al ha'Itur. If amidst coercion a person has Gemiras Da'as to sell, all the more so coercion gives Gemiras Da'as to buy. However, Kidushin is different. He does not monetarily acquire her; he just forbids her to everyone else. Letter of the law, it is valid whether he or she was forced. They argue about whether Chachamim uprooted it when he was forced.

Chasam Sofer (Bava Basra 47b DH v'Lavo): The Rambam and Ra'avad explain that Bava Basra (48b) discusses when he or she was forced.

Gra (2): (A man who is Mekadesh is like a buyer.) The latter opinion, Ba'al ha'Itur, holds that if one was forced to sell, it is valid. We did not say so about one who was forced to buy. The Rashba, Magid Mishneh and Ra'avad hold that all the more so, this is valid! The Ba'al ha'Itur rejects the Kal va'Chomer, for a seller receives money. In CM, the Beis Yosef and Rema [rule like Ba'al ha'Itur. They] overlooked that the Rambam, Ra'avad, Rashba and Magid Mishneh, Semag and other Poskim disagree. They are correct. If one was forced and received, even if he lost something worth more, it is valid. One should be quick to buy land [but not to sell - Yevamos 63a. Even so, a coerced sale is valid. Coerced Kidushin is valid, even though she gets a mere Perutah.] One who is forced to buy benefits more. Surely it is valid!