Gemara

(Mishnah): We may write a loan document for a borrower, even in the absence of the lender. We may not write for the lender in the borrower's absence.

(Rav Yehudah): If he says that the document is Amanah (the borrower trusted the prospective lender and gave to him the document before the loan was given), he is not believed.

(Rav Ashi): He discusses when the witnesses say that it is Amanah, and we cannot verify their signatures without them. They are not believed, due to Rav Kahana's law:

(Rav Kahana): One may not keep a document of Amanah in his house - "Do not let Avlah (fraud) dwell in your home".

(Rav Sheshes brei d'Rav Idi): Witnesses do not sign on Avlah.

Bava Metzia 13a - Question: Why may one write a document for the borrower in the lender's absence? Perhaps he will lend after the date of the document, and improperly take land from people who bought before the loan!

Answer (Rav Asi): The Mishnah discusses Hakna'ah documents, in which the borrower puts a lien on his land immediately (the collection is proper).

Sanhedrin 29b (Rav Nachman): It is normal for one to falsely claim that he owes money, so people will not think that he is rich.

Rishonim

The Rif and Rosh (10:18) bring our Mishnah.

The Rif and Rosh (Kesuvos 6a and 2:9) bring the Gemara in Kesuvos.

Rambam (Hilchos Malveh 23:5): We may write a loan document for a borrower, even in the absence of the lender, but not for the lender in the absence of the borrower. This discusses a document with a Kinyan, for from the time of the Kinyan the borrower's property is Meshubad. If there is no Kinyan, we may not write even for the borrower until the lender is also there and he gives the document to the lender in front of us. We are concerned lest he write to borrow in Nisan, and not borrow until Tishrei, and the lender will take property sold from Nisan. This is improper, for he did not receive the document until Tishrei.

Magid Mishneh: Some say that there is no Chidush that we do not write for the lender in the borrower's absence. It is merely for parallel structure with the Reisha. I say that it forbids even if the lender says 'write and sign the document, and it will be in your hands. If the borrower will come and say that it is true, acquire from him and give it to me. If not, tear it.' We forbid because it is false when it is written, and perhaps he is scheming.

Beis Yosef (CM 39 DH v'Ein): We would allow someone to borrow a second time with a document that was paid (if not because the lien was pardoned - Gitin 26b). We are not concerned that it looks false! We can say that here is different, for from when it was signed it looks false.

Rebuttal (Lechem Mishneh Edus 6:8): The Rif holds that we are not concerned for a validation of Beis Din that looks false. The Rosh says that we are concerned. Even the Rosh is concerned for this only for acts of Beis Din! Rather, the Magid Mishneh holds that perhaps the lender hopes that the witnesses will lose the document and he will find it, or another ruse. The Beis Yosef understood that the concern is that it looks false. Also his answer is wrong, for in Kesuvos (85a) we permit writing and signing a document for a woman before she swears; we are not concerned that it looks false. We compare this to signing a Get found in the trash. This shows that we do not distinguish whether it was false from when it was signed.

Poskim

Shulchan Aruch (CM 39:13): We may write a loan document for a borrower, even in the absence of the lender.

Beis Yosef (DH u'Mah she'Chosav Rabeinu u'Bilvad): Here, the Tur requires the witnesses to recognize only the borrower, like the Magid Mishneh (Hilchos Malveh 24:3). However, in Siman 49 he requires that they recognize also the lender.

SMA (21): We must recognize the lender, for perhaps the lender owes others and they scheme to write a different name in order that the lender's creditors will not be able to collect from this loan. Also, one does not want others to think that Ploni owes to him if it is not true. This is only when both are in Beis Din or in front of witnesses, for then they write 'he admitted to him'. However, if the borrower alone says 'write that I owe to Ploni', we need not suspect that he seeks to help the true lender evade a creditor, for one does not sin to help another. Also, then we do not write 'he admitted to him', just 'David admitted (that he owes to Ploni).' An admission not in front of the lender does not create publicity about how much he owes to him. Perhaps the borrower merely does not want people to think that he is rich.

SMA (49:3): If there is no Kinyan in the document, we must recognize the lender, lest the borrower plans to give it to the lender tomorrow, and the document will be predated.

Beis Yosef (DH u'Mah she'Chosav Rabeinu v'Einam): We may write a document for a borrower in the lender's absence only on condition to give it to the borrower.

Tur: If David said 'acquire from me that I owe this amount to Ploni; write a document and give it to me', they write it and give it to David. Ploni acquires only if David gives it to him. The witnesses may not directly give it to Ploni. When David commands to give it to Ploni, David's property is Meshubad from the time of the Kinyan.

Bedek ha'Bayis: They may not give it to Ploni, for David was not Meshabed himself until he gets the loan. If they gave it to Ploni, this is fraud.

Tur: If David said 'acquire from me that I owe to Ploni; write and give it to him', or even if he said Stam 'write', they write and give it to Ploni, and not to David.

Beis Yosef (DH u'Mah she'Chosav Oh): Stam Kinyan is destined to be written and given to the lender. This is even if he did not say 'write'.

Rema: However, if the lender protests that we not write for the borrower, we do not write Bal Korcho (against his will).

SMA (35): A man (can say that) he does not want people to think that he is owed, especially if it is not true.

Gra (58): We learn from Sanhedrin 29b.

Shulchan Aruch (ibid): We may not write for the lender unless the borrower is there. Even if the lender says 'write and sign the document, and it will be in your hands. If the borrower will come and say that it is true, acquire from him and give it to me. If not, tear it', we do not heed him.

SMA (36): The concern is only for signing it. We may write in the absence of the lender, like the Beis Yosef said (above, DH v'Ein).

Note: The Beis Yosef (Sof DH uvi'Teshuvos, citing Teshuvas ha'Rashba 945) says that if the lender says 'do not write and sign for Ploni (who accepted with a Kinyan that he owes me), we do not write for him.

Shach (38): The Ritva (Kesuvos 21b DH veha'Omar) says that we are concerned only about signing Sheker, but not writing Sheker. I disagree. We are not concerned even for signing. Tosfos and the Rosh (Kesuvos 85a DH Alma and 9:7) explicitly say so. A validation is different, for if the judges sign before they hear testimony, the validation is total Sheker. The Rashba and Ran (Gitin 26b DH Leisa and 13b DH Mihu) say that Bava Basra 172a (writing in a document the place where it was authorized, and not where it was written) is different, for even retroactively it is Sheker. Rather, the Bach explains that the primary reason is that here, since it is Sheker, we are concerned lest the lender schemes, so we do not write it. When the borrower asks us to write, there is no concern.

SMA (38): The Shulchan Aruch discusses stipulating about if the borrower will come the same day. If not, the document would be predated.