Gemara

(Mishnah): A predated loan document is invalid.

157b - Question: Granted, one may not use it to collect from (Meshubadim, i.e. land sold by the borrower after) the first date (what is written), but it should be valid to collect from the second date (the true date of the loan)!

Answer #1 (Reish Lakish): Indeed, Chachamim say that it may be used to collect from the second date. This Mishnah is R. Meir (who makes a fine that one loses what he is entitled to, to discourage people from trying to get what they are not entitled to (earlier Meshubadim).

Answer #2 (R. Yochanan): Chachamim admit in this case. This is a decree, lest he come to collect from the written date.

A case occurred, Reuven threatened Shimon 'if you do not sell your orchard to me, I will claim that it is mine'. (Reuven had a Chazakah, so he would have been believed.) Shimon was Makneh the orchard to his son (a minor), then Shimon sold it to Reuven. Clearly, the sale to Reuven is invalid.

Question: Is the money Reuven paid like a loan with a document (the Achrayus entitles him to collect from Meshubadim)? Or, is it like a Milveh Al Peh (a loan without a document), which cannot be collected from Meshubadim?

Answer (Abaye): We learn this from R. Asi's law!

R. Asi: If Levi admits that he authorized a document, Yehudah can use it against him, Levi cannot demand that he validate it. (Shimon admits that Reuven paid for the orchard, like the document says. It is invalid to acquire the orchard, but it shows that Shimon owes him money!)

Rejection (Rava): The case of the orchard is different, for there was no Heter to write the document!

Question (Ravina): If so, why does R. Yochanan say that it is a decree not to collect from the latter date, lest he collect from the date written? He should say the document is invalid because there was no Heter to write it!

Answer (Mereimar): A predated document had a Heter to be written, i.e. with the date of the loan. There was no Heter to write the document of the orchard at all!

Bava Metzia 17a (R. Yochanan): If a loan was paid, the document cannot be used for another loan (of the same amount between the same parties), for Nimchal Shibudo (the lien created against the borrower has been pardoned).

Question: If the second loan is on a later day, even without Nimchal Shibudo it is Pasul because the date precedes the loan!

Mishnah: Predated loan documents are invalid.

Rishonim

Rif and Rosh (Bava Metzi'a 42b and 5:58): R. Yochanan says that even Chachamim decree. This is the Halachah; it is like a Milveh Al Peh.

Rambam (Hilchos Malveh 23:1): A predated documents is invalid, for it allows the lender to fraudulently collect land from buyers. Chachamim made a fine that it may be used to collect only from Benei Chorin (property of the borrower). This is a decree lest one collect land sold after the earlier date.

Hagahos Maimoniyos (1): If the document says that it is predated, it is Kosher.

Question: Why is the document valid for Benei Chorin? The witnesses are Pesulim, for they intentionally signed Sheker!

Answer #1 (Rosh, ibid.): The case is, there are other witnesses who say that the witnesses who signed were coerced and signed to save their lives. Alternatively, they erred about the year of the king or the day of the month (they thought that that last Chodesh was 30 days), and the document does not mention the day of the week, only the day of the month.

Answer #2 (Nimukei Yosef DH Pesulim and Ran Rosh Hashanah 2a DH Shitrrei): A Mishnah teaches that we may write (and sign) a document with the proper date and gave it to the borrower. The witnesses did so; the loan was given later. Really, one may do so only if the borrower puts a lien on his property immediately. However, witnesses who do not know this are not Resha'im.

Answer #3 (Pirush ha'Mishnayos Shevi'is 10:5): The case is, the borrower paid up the loan and left the document with the lender. The lender later lent again, relying on the same document.

Rebuttal (Lechem Mishneh): Once a loan is paid, the document is totally Pasul. This is even if the date is correct, e.g. the second loan was the same day!

Defense (Shach CM 43:15 DH v'Ra'isi): The Rambam teaches that it is called predated even in such a case (when it was initially Kosher). He discusses a case in which it is not Nimchal Shibudo, e.g. the borrower gave it to the lender again at the time of the second loan. Edei Mesirah Karsei (the witnesses who see a document handed over empower it), so it is Pasul only because it is predated.

Answer #4 (Ro'oh, brought in Beis Yosef CM 43 Sof DH v'Yesh Omrim): The witnesses thought that the money was already lent when they signed. Later, they saw when it was lent, and testified about this.

Answer #5 (Gedulei Terumah, cited in Shach CM 43:13): Indeed, the Rosh (ibid.) cites Tosfos (Bava Metzi'a 72a DH Shtar), who says that the Gemara calls the document Pasul, which connotes that one may not collect with it at all.

Rebuttal (Shach, ibid.): If so, Chachamim would not need to decree to disqualify it lest one collect from the first time! Rather, the Rosh says that Tosfos totally disqualifies it even when the witnesses are Kesherim, e.g. like the Rosh answered.

Question: The Gemara asked why it is invalid to collect from the second date (i.e. from Meshubadim. Regarding Benei Chorin the date is irrelevant.) It should have asked why it is invalid to collect from Benei Chorin (and then the answer would clearly show that it is totally Pasul)!

Answer (Tosfos, ibid.): The Gemara teaches that even if the borrower admits that he did not pay or witnesses say so, the document may not be used to collect from Meshubadim.

Question: How can we know that a document is predated? The witnesses cannot tell us, for this is like retraction of their testimony!

Answer #1 (Ba'al ha'Ma'or Reish Rosh Hashanah): If a king took office in Adar, his first Nisan is called his second year. If a document is dated in Nisan of his first year, it is predated.

Answer #2 (Ba'al ha'Ma'or, ibid.): Regarding any mistake that witnesses commonly make, they themselves are believed to say 'we signed on this, but not on this' (Yerushalmi. Shach (43:19) - e.g. they say that they signed on the loan, but did not pay attention to the date.)

Hasagos ha'Ra'avad: All the more so, we believe other witnesses to say that the signed witnesses erred. Normally, we are not concerned lest two witnesses erred; I explained this elsewhere.

Answer #3 (Ramban Bava Metzi'a 72a DH veha'Me'ucharin, brought in Nimukei Yosef, ibid): They can say that it is predated when we could not validate the document without them. Reish Lakish taught that a signed document is like testimony. R. Meir holds that the witnesses cannot invalidate it, even if we could not validate it without them! That is when they say that it is totally invalid. Here they say that it is valid for Benei Chorin.

SMA (43:13): When we have no other way to validate it, they are believed to say that they erred, Migo (since) they could have said that it is not their signatures.

Answer #4 (Beis Yosef DH Aval): Other witnesses are believed to say that it is predated even if we can validate it.

Poskim

Shulchan Aruch (CM 43:7): Chachamim made a fine that a predated document may be used to collect only from Benei Chorin.

Shulchan Aruch (8): The document is valid for Benei Chorin only if the witnesses did not intentionally predate it.

Rema: Some say that witnesses are believed to say that they predated it ,even if their signature is known (we could validate the document without them). Similarly, regarding any mistake that witnesses commonly make, they are believed to say that they erred. Some disagree.

Shach (19): The first opinion, the Ba'al ha'Ma'or, holds that even if their signature is known, they are believed even about mistakes we cannot attribute to error. They can say that they overlooked the date. The latter opinion, Tosfos and the Rosh, holds that they are not believed when their signature is known. They do not discuss whether they are believed about a common mistake in this case. Perhaps we need other witnesses to say that the document is predated! It seems that they are believed like the Ramban explicitly says; we need not make another argument.

Shulchan Aruch (ibid.): If they intentionally predated it, the witnesses are Pesulim for signing Sheker, and we do not collect even from Benei Chorin.