Gemara

(Mishnah): If Reuven presents a document in Shimon's handwriting saying that Shimon owes him, he collects from Bnei Chorin (i.e. but not from property that Shimon sold).

Gitin 86a (Mishnah): The following Gitin are disqualified, but if she remarried, the children are not Mamzerim:

It is written in the husband's own handwriting, witnesses did not sign it...Witnesses signed it, there is no date on it.

R. Elazar says, even if no witnesses signed on it, but it was given in front of witnesses, it is valid, and it may be used to collect from sold land.

Bava Basra 167a (Abaye): If someone wants Beis Din to have his signature (to validate documents), he should not sign at the bottom of a parchment, lest a swindler will find it, and write above it 'I owe you (a large amount of) money';

(Mishnah): If Reuven brought a document against Shimon, and Shimon was signed below, Reuven collects from Shimon's Bnei Chorin.

175b - Question (Rabah bar Noson): If Beis Din validated a document in the borrower's Kesav Yado, what is the law?

Answer (R. Yochanan): The lender collects only from Bnei Chorin.

Rishonim

Rif and Rosh (Bava Basra 82b and 10:16): R. Yochanan answered that even if Beis Din validated a document bi'Chsav Yado, it may be used to collect only from Bnei Chorin. This is because it is like a loan through witnesses (without a document). If Shimon admits that he owes, Reuven collects from Bnei Chorin. If Shimon says that he paid, he swears Heses and is exempt. If Shimon denies that he borrowed and wrote the document, if his handwriting was established in Beis Din or witnesses recognize it, he is established to be a denier, and he pays. If one denies borrowing, he says that he did not pay.

Rebuttal (Ba'al ha'Ma'or): The Rif says that Kesav Yado is like a Milveh Al Peh, and if Shimon says that he paid, he swears Heses and is exempt. This is not clear. Reuven can say, if you paid, why do I still have the document? Rav Papa says so about a proper document. It seems that the same applies to Kesav Yado. The Mishnah said that one collects from Bnei Chorin with Kesav Yado. It did not say that the borrower is believed to say that he paid!

Defense (Milchamos Hash-m): Also when there are witnesses (a Milveh Al Peh), the Mishnah says only that he collects from Bnei Chorin, without specifying that he is believed to say that he paid! Since it says the same about both of them, this implies that their laws are the same. A borrower is not careful to take back the document when he pays. He is careful to take back a document on which witnesses signed, since it creates a lien and cheapens his property (people are reluctant to buy it).

Rambam (Hilchos Malveh 11:2): If the borrower wrote a document bi'Chsav Yado and witnesses testified on it and he gave it to the lender, it is a Kosher document. Similarly, if he wrote a document, even if there are no witnesses on it and he gave it to the lender in front of witnesses, this is a loan with a document. This is if the writing cannot be forged and the witnesses who saw it given read it. Some Ge'onim ruled that he must tell those witnesses to sign and testify that it was given in front of them.

Rambam (3): If Reuven presents a document in Shimon's handwriting saying that Shimon owes him and there are no witnesses, even if his handwriting was established in Beis Din it is like a Milveh Al Peh in every respect. If Shimon says that he paid, he swears Heses and is exempt. Kesav Yado may not be used to collect from heirs or buyers.

Rambam (Hilchos To'en 6:3): If Reuven presents a document in Shimon's handwriting saying that Shimon owes him, and Shimon says that he did not borrow or write this, if his handwriting was established in Beis Din or witnesses recognize it, he is established to be a denier, and he pays.

Poskim

Shulchan Aruch (CM 40:2): If the borrower wrote a document bi'Chsav Yado and witnesses testified on it and he gave it to the lender, it is a Kosher document.

Beis Yosef (DH v'Nir'eh): R. Tam says that if a borrower wrote in his handwriting 'I owe to you 100' and gave it in front of witnesses, it is a loan with a document. It is no worse because it is in his handwriting. Even though the text is unlike a document, it suffices because it contains an admission and was given in front of witnesses.

Shulchan Aruch (69:1): If Reuven presents a document in Ploni's handwriting saying that Ploni owes him, he collects from Bnei Chorin. This is whether he wrote 'I, Ploni who signed below admit that I owe 100 to Reuven' and he signed below, or whether it is in another's handwriting and Ploni signed at the bottom. The same applies if he did not sign at the bottom, but just wrote 'I, Ploni ben Almoni owe to you 100.' However, we distinguish. When he signed below, even if the entire top of the document is in another's handwriting Reuven collects. When he signed above, Reuven collects only if everything is in Ploni's handwriting. We are concerned lest Ploni signed at the top of the page and someone else found it and wrote underneath. When it is entirely in Ploni's handwriting we are not concerned for forgery. Even if it does not say Ploni's name, just 'I owe to Reuven this amount', he must pay. Therefore, receipts that partners bring against each other, even if they are not signed, they just say 'I received this much on this day', may be used to collect from Bnei Chorin. The one who wrote it cannot say 'I never received.'

Beis Yosef (DH Mi, citing Ba'al ha'Terumos): Some say that 'Kesav Yado' always refers to when he signed his name at the bottom. This is wrong. It suffices if the entire document is in his handwriting. However, if he signed above and a debt is written below in another's handwriting this is not called Kesav Yado. It does not help, for perhaps someone forged it. We learn from Bava Basra 167a, which says not to sign one's name at the bottom of an empty parchment lest someone write above it. (One may sign at the top without concern.) Rav Huna was Machshir a document that began with the borrower's name (172a) because it was entirely in the borrower's handwriting. Therefore, it did not need a signature at all. The Ramban proves that such a document is Kosher from Gitin 86a. A Get in Kesav Yad ha'Ba'al without any witnesses is Kosher (mid'Oraisa). The Yerushalmi says that it may be used to collect from Bnei Chorin. It cannot mean that he signed it, for it is taught with Kesav Sofer, in which the scribe wrote it but did not sign it.

Beis Yosef (DH v'Zeh): The Ritva (Kesuvos 102a DH ul'Inyan) says that Rashi holds that if Kesav Yado that was not signed was handed over in front of witnesses, it is valid according to R. Yochanan, and the Halachah follows him. If it was not handed over in front of witnesses, R. Tam and the Ba'al ha'Itur say that it is invalid. However, the Ramban says that it is valid. It seems that if the one who wrote it says that it was a standard form like scribes write and he never gave it, it is invalid. However, it seems that we rely on receipts that partners write to each other.

Beis Yosef (DH v'Zeh Lashon ha'Rashba): The Rashba (3:55) says that if an entire document is written in Kesav Yado, we learn from the bottom line, because there is no concern lest the bearer wrote it himself. Even if he did not sign it he is liable. The signature does not obligate, rather, the admission. This is why the Gemara discusses Kesav Yado, not Chesam Yado.

Rema: Even if he wrote in his handwriting 'my signature below testifies against me like 100 witnesses', it is like a regular Kesav Yado.