Gemara

(Mishnah): We may write a document for the borrower in the lender's absence.

Question: Why is this l'Chatchilah? Perhaps he will lend the money after the date, and the lender will improperly take land from people who bought before the loan!

Answer #1 (Rav Asi): The Mishnah discusses Hakna'ah documents, in which the borrower puts a lien on his land immediately (so the collection is proper).

Question: The Mishnah says that one who finds a document with Achrayus should not return it. We said that this is when the borrower admits that he owes. We are concerned lest the loan was given after the date on the document, and the lender will collect improperly from property sold before the loan;

If it is a Hakna'ah document, he is entitled to collect. If it is not a Hakna'ah document, it is written only in front of the lender. Surely, the loan was the same day!

Answer (Rav Asi): The Halachah is, a document without Hakna'ah should be written only in front of the lender. Since this document was lost, we are concerned lest it was written (improperly) in the lender's absence.

Answer #2 (to Question (a) - Abaye): Even if a document is not Hakna'ah, when witnesses sign, the lender immediately acquires (a lien).

Bava Basra 172a - Rava bar Rav Shila (to scribes): When you write an Hakna'ah document (in which a Kinyan Chalipin was done), if you know the date of the Kinyan, write it. If not, write the current date, lest it look like Sheker.

Kesuvos 102a (Mishnah): If Levi married Leah and agreed to feed her daughter Dinah for five years, he must feed her for five years. If Levi died, his own daughters are fed from Benei Chorin. Dinah is fed even from Musafim (property sold after he accepted to feed her), because she is like a creditor.

102b: The case is, the commitment was acquired through Chalipin.

Rishonim

Rif (Bava Metzia 6a): We establish the Mishnah to discuss Hakna'ah documents. I.e. it has a Kinyan, in which the borrower immediately was Meshabed himself to the lender for a certain amount. If there was no Kinyan, there is no Shibud until the document is written.

Rebuttal (Rosh 1:36): Since the document says that Ploni borrowed from Ploni, why does it make a difference whether or not there is a Kinyan? Shibud is mid'Oraisa! Whatever one owes, there is a lien on his property to pay it! Witnesses who sign make a Kol, not witnesses who saw a Kinyan! Rather, Rashi explained that in a Hakna'ah document, the borrower was Makneh his property to the lender. Whether or not he will lend, he may collect from the date on the document. It is astounding that one was Meshabed his property before the loan. This causes a Kol from when it is written.

The Rif (Bava Basra 79b) brings Rava bar Rav Shila's teaching.

Nimukei Yosef (DH Omar): The Ri says that a Kinyan is destined to be written, therefore there is a Kol from the time of the Kinyan. However, if they do not write the document, the Kol is Batel. Ba'al ha'Itur and the Ro'oh say that even loan documents can be written from the time of the Kinyan, unless it is now after the loan was due, for then we are concerned lest it was paid. The Rashba says that we may write as long as the borrower does not protest.

Rambam (Hilchos Malveh 23:5): If a document has a Kinyan, we may write it for the borrower in the lender's absence, for the borrower was immediately Meshabed his property. If there was no Kinyan, we may write only when the lender is here, and we will see the borrower give the document to him, lest it be given later and he will be Toref from the date, improperly.

Rosh (Gitin 2:7): We disqualify a document signed the night after it was written when there was no Kinyan. If there was a Kinyan, there is no concern. We say in Bava Basra that witnesses may write a document from the day the Kinyan was made, if they remember it.

Rosh (Bava Basra 10:31): The document is not predated. Since the document was written, witnesses publicize it from the day of the Kinyan. Normally, people borrow covertly. Here, he generously was Meshabed his property, so there is a Kol. In Gitin, we disqualify a document signed the night after it was written. This is when there was no Kinyan.

Rashbam (172a DH l'Hanach): Rav Shila permits only documents of gift or sale, but a loan document may be written only on the current day. One may be Toref from buyers only from the day it is written. If the day of the loan is written, the lender can be Toref illegally. R. Chananel explains that the Shibud is from the time of the Kinyan. What I wrote is correct.

Poskim

Shulchan Aruch (CM 39:1): If witnesses say that he made a Kinyan (in which the borrower was Meshabed his property to the lender), even if it was not written, he collects from Meshubadim.

Gra (4): We find in Kesuvos 102b that Kinyan enables collecting from Meshubadim. The Poskim hold that Shitrei Aknaisa, even for loans, enable collecting from Meshubadim, unlike the Rashbam. Bava Metzia 13a supports this, according to the Rif and other Poskim.

Shulchan Aruch (43:16): If a document was written during the day and signed at night, it is Pasul. However, if he immediately acquired through a Kinyan it is Kosher, even if it was signed only much later. Therefore, if witnesses saw the Kinyan and wrote the document much later, if they remember the date of the Kinyan, they write that day. If not, they write the current day.

Beis Yosef (DH v'Davka): The Rosh and Ran learn this from Bava Basra 172a. The Rashbam explains that a loan document may be written only on the current day. If the day of the Kinyan is written, the lender can be Toref illegally. R. Chananel explains that the Shibud is from the time of the Kinyan. It seems that the Rambam agrees, for he did not distinguish.

SMA (39:2): A Kinyan Sudar (Chalipin) in front of witnesses shows that he is not concerned for publicity. The witnesses will publicize it, even if no document was written. This is why the Shulchan Aruch concludes that Kinyan suffices, unlike the Nimukei Yosef and Rashbam. This is only if the witnesses are here and remember when the Kinyan was. If not, even if the borrower admits that he borrowed on this date, and other witnesses saw the Kinyan, he cannot collect from Meshubadim, for only the witnesses who did the Chalipin publicize the matter. We must be concerned lest they forgot the matter, so a dated document is needed to collect from Meshubadim.

Rebuttal (Shach 35 and Gra 29): He collects from Meshubadim even if no document was ever written, for we hold that Kinyan has a Kol, like the Shulchan Aruch said (39:1). This is like most Poskim, and unlike the Rosh.

Taz: In Siman 39, the Shulchan Aruch allows collecting from Meshubadim without a document. This refers to within the time (before the loan was due). Here we discuss after the time, and witnesses signed before the time.