Rambam - 1 Chapter a Day

I.e., he threatened his life itself.

A sale is dependent on the seller’s consent. Nevertheless, if he issued such consent, even if he was compelled to do so, it is binding and the sale is considered to be unalterable.
In his gloss on Halachah 8, the Maggid Mishneh states that this principle applies with regard to a sale, but not to a gift.

See Shulchan Aruch (Choshen Mishpat 205:4), which discusses whether there is a difference between landed property and movable property if the person does not pay the entire worth of the object that he takes.

But made an admission to witnesses that he took the funds in exchange for the field.
The Ra’avad and the Tur (Choshen Mishpat 205) maintain that for this law to apply, it is necessary for the witnesses to see the money pass hands. The Shulchan Aruch (Choshen Mishpat 205:1) quotes the Rambam’s view, while the Ramah follows that of the Tur.

Since the sale is dependent on the will of the seller, if he makes an explicit statement to the fact that the sale is against his will, the sale is nullified.

Our translation is somewhat loose. Sefer Me’irat Einayim 205:6 offers two explanations for the term moda’ah:
a) information - the seller informs the witnesses of the factors compelling him to sell;
b) suffering - he notifies them that he is undergoing suffering and he is therefore selling the property.

Ordinarily possession is considered to be proof of ownership of movable property immediately, and of landed property after three years. Nevertheless, these principles do not apply with regard to a thief. He is never considered to be the legitimate owner.

They need not know in which manner he is being compelled against his will (Beit Yosef, Choshen Mishpat 205).

I.e., a person came with a claim and the defendant denied it, and rather than engage in a protracted dispute, each agreed to forgo a portion of their claim.

For he is not receiving anything in return. Therefore, even though we do not see that he is being compelled against his will, since he does not desire to give up his ownership of the article being given as a gift, it remains his property.
With regard to a sale or a compromise, by contrast, even if the seller originally protested, since he is receiving money for the sale or avoiding difficulty by the compromise, we assume that he agreed unless it is obvious that he is under compulsion.

See Hilchot Zechiyah UMatanah 5:4, where this concept is explained in depth.

For like a person giving a gift, the creditor does not receive anything in exchange for his gesture of good-will.

The Ramah (Choshen Mishpat 205:7) cites an opinion that states that mere threats are not considered compulsion. For people realize that at times people make threats without carrying them out.

I.e., as the Rambam continues to explain, we are not speaking only of physical compulsion, but also about financial compulsion. Similarly, blackmail concerning one’s reputation could also be considered a relevant factor.

I.e., the rental contract; the commentaries question the use of this wording.

This time period is significant. As the Rambam explains (Hilchot To’en V’Nit’an 11:1-2), after landed property has been in a person’s possession for three years, we accept his claim that he purchased it although he cannot present a bill of sale showing that he acquired it from the previous owner.

Since the tenant had been in possession of the field for three years, his claim would be accepted by the court.

I.e., the danger of denial was not merely a potential, but was actually brought into expression in court.

Telling the witnesses of the threat the tenant made.

From the Rambam’s words, it would appear that the same witnesses must be aware of the compulsion and hear the protest. The Maggid Mishneh, however, explains that as long as a protest is issued before the sale, it is effective, even if the seller proves he was acting under compulsion through other witnesses.

I.e., when is a protest necessary?

Chamas, the root of chamsan, denotes crime, injustice, violence and outrage. The Maggid Mishneh explains that if the seller willingly accepted the money that he was offered, despite the fact that originally he was forced to sell against his will, the purchaser is not considered a chamsan. Only when the seller desires to protest is the title applied to the purchaser. Despite the fact that he pays, he is still considered to be acting criminally.

I.e., it became public knowledge that the robber had taken the field from its rightful owner.

In Hilchot Gezelah 9:14-16, the Rambam writes:
[The following rules apply when a person] obtains a field by robbery, and after it was established that his intent was to take it by robbery, he then purchases it from the original owner. If the [original] owner claims: “I was acting under coercion at the time that I sold it to him. I sold it to him against my will, because he was a robber,” the robber does not acquire [the field], even though there are witnesses that he purchased [the field] in their presence. Instead, the field should be restored to its [original] owner and the robber should be given back the money that he paid....
The owner of the field does not have to issue a protest over the sale in such an instance, for it has been established that the field was taken by robbery, and any proof of ownership brought by the robber is not accepted.
The Ra’avad, Rabbenu Asher, the Tur and the Ramah (Choshen Mishpat 151:3) differ with the Rambam’s decision in Hilchot Gezelah. They maintain that if the witnesses to the sale saw the robber give the owner money and the owner does not issue a protest, the sale is binding.
In his Kessef Mishneh, Rav Yosef Karo explains the Rambam’s decision, stating that it depends on the distinction between a person who was already established as having taken a field by robbery, and other instances when a person was compelled to sell a field against his will. And in his Shulchan Aruch (Choshen Mishpat 151:3, 205:8) he quotes the Rambam’s words.

I.e., we do not say that their signature indicates that the seller voluntarily consented to the sale.
Ordinarily witnesses cannot give testimony that negates a legal document that they signed. Nevertheless, in this instance, the protest that they signed precedes their signature on the deed of sale. Therefore, it is acceptable.

Instead, we assume that the robber took the field without paying for it and compelled the former owner to say that not only did he sell it willingly, but he received money as well.

For then it is obvious that money was in fact paid for the property. It should be returned to the robber.

I.e., the seller’s present statements indicate that he is willing to carry on with the sale. The Maggid Mishneh and the Kessel Mishneh emphasize that even if the seller is compelled to nullify the protests under duress, the nullification is binding. Just as a sale made under duress is binding, a nullification made under duress is binding. The only alternative is to issue a protest against the nullification, as the Rambam continues to explain.
See also Hilchot Gerushin 6:20, where the Rambam speaks about the potential to issue a protest against a bill of divorce. That protest can be nullified in the following manner:
The witnesses should tell [the husband] before the composition of the get: “Affirm in our presence that any statements that you have made which, when verified, would cause [this] get to be nullified, are themselves nullified.” [The husband] must answer “Yes.”

The Kessef Mishneh notes that in Chapter 5, Halachah II, the Rambam writes that a kinyan has no effect with regard to the nullification of a protest. He explains, however, that, as the Rambam explains in Chapter 5, Halachah 13, the kinyan is effective in demonstrating that the person is not making the statements facetiously and genuinely desires to nullify the protests.

I.e., the protest he issued against the nullification of the protests is heeded. Despite the fact that he said he wished to nullify his protest, we assume that the statement is being made against his will.
The Rambam’s words are quoted by the Shulchan Aruch (Choshen Mishpat 205:11). The Tur and the Ramah maintain that even in such a situation if one explicitly states that one negates all protests, the protests are nullified. See also Sefer Me’irat Einayim 205:27, which quotes the suggestion of the Rashba, who states that one can nullify the protests by stating that one disqualifies the witnesses before whom one issued the protests.