Gemara

Mishnah: Predated loan documents are invalid. Postdated documents are valid.

Bava Metzi'a 72a - Question: Why are predated documents invalid? Granted, they may not be used to collect from (Meshubadim, i.e. land sold by the borrower after) the first date (what is written), but they should be valid to collect from the second date (the true date of the loan)!

Answer #1 (Reish Lakish): Indeed, Chachamim say that they 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, i.e. earlier Meshubadim).

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

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

Question: What is the case?

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 (a loan without a document).

Rosh (ibid., citing Tosfos Bava Metzi'a 72a DH Shtar): The Gemara calls the document Pasul, which connotes that one may not collect with it at all.

Gra (CM 43:7 DH v'Yesh Omrim): Reish Lakish compares this to a document that specifies Ribis. A Tosefta teaches that the latter is totally Pasul.

Rambam (Hilchos Malveh 23:1): Predated documents are invalid, because they allow the lender to fraudulently collect land from buyers. Chachamim made a fine that they may not be used to collect property that the borrower sold. This is a decree lest one collect land sold after the earlier date.

Poskim

Shulchan Aruch (CM 43:7): Predated documents are invalid, because they allow the lender to collect fraudulently. Chachamim made a fine that they may not be used to collect from Meshubadim. This is a decree lest one seize land sold after the earlier date. If the borrower claims that he paid, it is as if there is a valid document against him. If he denies that he ever borrowed, he is established to be a false denier.

Beis Yosef (DH v'Im): The Rosh said that it is like a Milveh Al Peh, then he cited Tosfos who totally disqualifies the document. The Rosh did not decide. The Tur holds like Tosfos, for he brings Tosfos' opinion last.

Bedek ha'Bayis: This does not prove that the Tur holds like Tosfos. Since the Rif, Rambam and Rashi agree, the Halachah follows them (it is a Milveh Al Peh).

Shach (10): If the borrower does not pay immediately from his property, Beis Din writes a Pesak Din which authorizes the lender to collect from property sold from now and onwards. We do not leave the predated document with the lender.

Shach (11): The SMA says that it does not help to write on it the true date of the loan. One could say otherwise. Perhaps the Gemara discusses a document on which there was no room to write anything else. There are places where we must say so. However, the law seems correct, since the document appeared in Beis Din when it was Pasul. Had the lender written on it 'this may be used to collect only from (the proper date, e.g.) Nisan 4', it would be Kosher.

Aruch ha'Shulchan (7): If the document says before the last line that it was signed some days after it was written, it is Kosher. The lender would not erase this, for the erasure would be evident.

Ramah (cited in Tur): The fine is only when a loan was given in Nisan and it was dated Tishrei, for there was no Heter to write such a document. But if a document was dated Nisan for a loan (intended) to be given in Nisan but it was not given until Tishrei, the document is Kosher to collect from land sold after Tishrei.

Rebuttal (Tur): We do not distinguish. Even though there was a Heter to write it, it could be used to collect fraudulently from Nisan.

Defense of Ramah (Taz DH v'Ein): Ravina (72b) asked why R. Yochanan said that it is a decree not to collect from the latter date, and did not disqualify because there was no Heter to write the document. According to the Tur, R. Yochanan needed to teach about the decree to disqualify even a document like this for which there was a Heter to write it!

Defense of Tur (Maharshak, Hagahah in Taz, ibid.): Indeed, Ravina asked why R. Yochanan explained that the Mishnah is a decree and disqualifies in every case, and did not say that the Mishnah disqualifies only when there was no Heter to write it!

Question (Taz, ibid.): The Tur himself rules like Abaye, that Edav b'Chosamav Zachin Lo (the Shibud begins immediately, even before the loan), so the document is totally Kosher! It is difficult to say that the Tur merely asks against the Ramah (because the Tur connotes that he himself disqualifies).

Rema: Some say that it is totally invalid.

Prishah (6): Here, the Tur says that it is totally invalid; this is not literal. In Siman 45, he cites R. Yonah who says that it is Pasul, i.e. that the borrower is believed to say that he paid it, but he cannot deny that there was a loan. Presumably, also here this is the Tur's intent. This is unlike the Rema, who says (Darchei Moshe 2) that the borrower is believed to say that he never borrowed.

Rebuttal (Taz DH v'Yesh Omrim): Here, the Tur cites the Ri, who holds that it is totally Pasul. R. Yonah is a third opinion. The Tur brings R. Yonah only to teach about a document with too much blank space above the witnesses.

Support (Rivash, brought in Shach 12): R. Yochanan (Bava Metzi'a 17a) taught that the same document cannot be used for a second loan after the first was paid. The Gemara says that he cannot discuss a loan on a later day, for even without Nimchal Shibudo, the document is Pasul because it is predated. Predated loan documents are invalid!

Defense of Prishah (Hagahah in Taz, ibid.): If the Tur held that it is totally Pasul, why does he discuss what the case is? He could simply say that the lender admits that it is predated; whether or not the witnesses are disqualified, the borrower pays only if he admits! Rather, the Tur seeks a case where the witnesses are not disqualified, for then the borrower cannot deny the loan.