Gemara

40a (Chachamim of Neharda'a): If the witnesses on a Moda'ah (a document saying that the seller is coerced to sell) did not write 'we know the Ones', it is invalid.

138b (Beraisa - R. Meir): If a Shechiv Mera said 'Ploni owes me 200 Zuz', the witnesses write this, even though they do not know whether it is true. Therefore, the heir must bring proof to collect it.

Chachamim say, the witnesses may not write this unless they know that it is true. Therefore, the heir collects it without proof.

(Rav Nachman): The opinions must be switched. R. Meir forbids writing if they do not know, lest a Beis Din err (and not require the heir to bring proof). Chachamim permit, for they are not concerned.

(Rav Dimi of Neharde'a): The Halachah is, we are not concerned lest a Beis Din err.

Question: Why is this different than Rava's law?

(Rava): Beis Din oversees Chalitzah only if they recognize (that she is a Yevamah and that this is her Yavam). Beis Din oversees Mi'un (a girl annulling her marriage) only if they recognize (the couple, and that she is married only mid'Rabanan). Therefore, witnesses (who see Chalitzah or Mi'un in front of Beis Din) may write a document of Chalitzah or Mi'un even if they do not recognize.

Inference: We require Beis Din to know because we are concerned lest another Beis Din err.

Answer: Regarding Chalitzah and Mi'un, we are concerned lest (a second) Beis Din will not check for themselves. Rather, they will rely on the Beis Din that witnessed it. We are not concerned lest a Beis Din rely on witnesses (e.g. who signed a gift of a Shechiv Mera). Surely, Beis Din themselves will verify what is written.

155a (Reish Lakish): There is a Chazakah that witnesses do not sign unless the parties in the document are adults.

Yevamos 106a (Rava citing Rav Huna): We oversee Chalitzah or Mi'un, even if we do not recognize the parties. Therefore, witnesses may write a document of Chalitzah or Mi'un only if they recognize the people.

(Rava himself): We oversee Chalitzah or Mi'un only if we recognize. Therefore, witnesses may write a document even if they do not recognize them.

Rishonim

Rif and Rosh (8:54): The Halachah is, we are not concerned lest a Beis Din err.

Nimukei Yosef (21a DH Lav): We say that if the witnesses did not know the Ones, the Moda'ah is invalid. We did not say that they may not write it. The Rashba and Ran derive that they may write it without knowing, and if later we learn about the Ones, the Moda'ah is valid. R. Yonah forbids to write without knowing. We are concerned lest people say that there is a Chazakah that witnesses do not sign (until they know that the document is true, just like sign only if the parties in the document are adults).

Rambam (Hilchos Mechirah 10:2): Witnesses who sign a Moda'ah must know that he sells due to Ones. They may not rely on what the seller says.

Rosh (Yevamos 12:18): The Halachah follows Rava, that we oversee Chalitzah or Mi'un even if we do not recognize.

Hagahos Ashri: R. Simchah says that in pressed circumstances, if the Yavam wants to go overseas, and we do not find witnesses who recognize, we rely on Rav Huna and do Chalitzah even without recognizing, and afterwards we seek witnesses who recognize. The Get Chalitzah is only for a proof. Witnesses of Chalitzah permit her, and enable her to collect her Kesuvah.

Poskim

Shulchan Aruch (CM 205:5): Some say that even if one said 'write for me a Moda'ah, and when it will become known through witnesses that I was forced, your testimony will join to nullify the sale' we do not heed him.

Beis Yosef (DH v'Chosav): The Magid Mishneh (10:2) brought only the Rashba, who permits writing a Moda'ah without knowing the Ones. Even R. Yonah agrees that if the Moda'ah was written and afterwards we learned about the Ones, the Moda'ah is valid. He only forbids writing it l'Chatchilah, lest Beis Din not require proof of the Ones.

Gra (14): This is difficult, for we say that Beis Din will rely on another Beis Din, but they will not rely on witnesses! See 255:1.

Rema: If they wrote a Moda'ah, and later the Ones became known, the sale is Batel.

Shulchan Aruch (255:1): If a Shechiv Mera (Reuven) said 'Ploni owes me 100 Zuz', the witnesses write 'Reuven commanded such and such', even though they do not know whether or not it is true. Therefore, the orphans must bring a proof in order to collect.

Tur: The Ramah says that witnesses may write without knowing only when he commanded to whom to give the money. They testify only about the command, and not on the claim that Ploni owes to Reuven. If he said only 'Ploni owes me 100 Zuz', the witnesses may not sign this (unless they know). I say that we do not distinguish. In either case they may write.

SMA (1): The Shulchan Aruch holds like the Tur. Even though the heirs must bring a proof, the document helps, for Ploni is unlikely to be brazen and deny when there is a written command. If nothing is written, he is brazen, for perhaps the witnesses will go away or die before the case comes to Beis Din.

Prishah (1): The Ramah holds that surely we discuss when he commanded to whom to give it. This is the command of a Shechiv Mera! The Tur learns from the Beraisa, which says only 'Ploni owes me 100 Zuz.' Also, witnesses may write without knowing because we are not concerned lest Beis Din err. This is even if he said only 'Ploni owes me 100 Zuz'!

Bach (DH u'Mah): The Ramah holds that surely we discuss when he commanded to whom to give it. If not, why would R. Meir permit writing it?! Chachamim are concern lest Beis Din err. Why should he argue, since there is no benefit from writing! Also, why does it say 'when he collects'? It should have said only 'he must bring a proof'! Rather, we discuss a Shechiv Mera who commanded to give to Almoni. R. Meir says that we write, in order that Almoni will be able to collect. We do not annul his rights due to concern lest Beis Din err.

Panim Me'iros (2:2): Reuven told witnesses 'write and give a document to Shimon, so he will have testimony about the house of our brother-in-law Levi; we legally bought half of it. That half belongs to Shimon (I sell my share to him). Levi's creditor says that Reuven has no proof that he bought from Levi. Others say that the witnesses testify to everything in the command, including that they bought from Levi. I say that the witnesses only say that Reuven said all this and gave his rights to Shimon. They do not testify about anything else. Levi's admission does not help. If something is Muchzak to a borrower, the Poskim agree that he is not believed to say that it is not his (so his creditor cannot take it). This is unlike witnesses who say 'Yosef the Kohen told us...', which connotes that they know that Yosef is a Kohen (CM 49). 'The Kohen' is not part of Yosef's words. A proof is from Moda'ah. If the witnesses did not write that they know the Ones, it is invalid (unless we can verify the Ones). We do not say that since they signed, presumably they knew the Ones he claims is true! Also Bava Basra 138b concluded that one must bring a proof to collect. We do not assume that the witnesses themselves verified what is written. Why do the Tur and Shulchan Aruch bring R. Yonah, who does not allow writing a Moda'ah without knowing the Ones? The Beis Yosef, SMA, Levush and Bach say that this is like Rava, who is concerned lest Beis Din err. How did all these Poskim err? The Gemara explicitly said that we are concerned lest a Beis Din rely on another Beis Din, but we are not concerned lest a Beis Din rely on witnesses! R. Yonah is not difficult. Perhaps he holds like the Ramah. Why does the Shulchan Aruch (205) bring R. Yonah's opinion, when in Siman 255 it holds that Beis Din will not rely on witnesses?!

Pischei Teshuvah (205:6): Sha'ar ha'Mishpat (2) says that the Shitah Mekubetzes (40b DH v'Zeh) explains that R. Yonah holds that we should not write a document to weaken the sale, for the sale was Vadai, and we are unsure about the Ones. The Moda'ah will cheapen the land in the eyes of people, for they will assume that it was not written for nothing. The concern is not lest Beis Din err! The SMA (238:5) explains in this way why we do not write a document of sale for the buyer if the seller is not here.