Gemara

Bava Metzia 17a (R. Asi citing R. Yochanan): If one finds in the market a validated document for a loan given that same day, he returns it to the lender.

We are not concerned lest the loan was never given, for it was validated. We are not concerned lest it was paid, for people do not pay loans on the same day.

Question (R. Zeira): R. Yochanan taught that if a loan was paid, the document cannot be used for another loan, for its lien was pardoned;

R. Yochanan did not need to teach this if the latter loan was on a later day. Even if the lien was not pardoned, the document could not be reused because the date precedes the loan. A predated loan documents is invalid!

Bava Basra 168a (Mishnah - R. Yosi): If Reuven paid part of his debt to Shimon, and gave the document to Levi and said 'if I do not pay the rest by date Ploni, give the document to Shimon (who may then collect the full amount)', and Reuven did not pay in time, Levi gives the document to Shimon.

Rishonim

Rambam (Hilchos Malveh 14:8): A document that was paid is a shard.

Perush ha'Rosh (Nedarim 27b DH R. Yosi): R. Yosi allows collecting the entire loan from Bnei Chorin. He cannot collect from Meshubadim, for the lien was pardoned. He cannot collect from Meshubadim even the part that was not paid, lest he collect the entire loan from Meshubadim. This is like a predated document. One may not collect from the true date, lest one collect from the date on the document. Here, he can collect from Bnei Chorin, for he acquired the document through Mesirah (handing it over). Alternatively, he can collect the entire debt from Meshubadim. He stipulated from the beginning that if he will not pay in time, the partial 'payment' was a gift, and the debt will remain like it was. This Perush is better, for one acquires a document only through Kesivah and Mesirah (writing another document and giving it over).

Ramban (Bava Basra 168a DH Masnisin) and Ran (Nedarim 27b DH She'ani): How does R. Yosi allow collection with the document? Its lien was pardoned! We must say that he stipulated from the beginning that if he will not pay in time, the partial 'payment' was a gift.

Poskim

Shulchan Aruch (CM 48:1): If a loan document was paid, one may not use it to borrow again, even if this is on the same day, for its lien was pardoned.

Ir Shushan (cited in SMA 2): If one cannot collect from Meshubadim with a signed document, one cannot use it even for Bnei Chorin. Since the testimony is partially Batel, it is totally Batel.

Rebuttal (SMA 2): We learn from one who paid part of his debt and said that if he does not pay by a certain date, the lender may collect the entire loan. The Rosh says that he collects the entire loan from Bnei Chorin. He cannot collect even the part that was not paid from Meshubadim, since the lien was pardoned. It seems the Rosh allows collecting, from Bnei Chorin, the part that was already paid only if there was through Kesivah and Mesirah. However, the Shulchan Aruch and Rema (55:1,2) allow collecting without this.

Defense (Shach 2): In R. Yosi's case, the borrower admits. Also, the Rosh concludes that he stipulated from the beginning that if he will not pay in time, the partial 'payment' was a gift, and the lender collects from Meshubadim. He also implies that without Kesivah and Mesirah, the first document cannot be used to collect (again the part that was paid) even from Bnei Chorin. The Ramban and Ran agree.

Shulchan Aruch (ibid.): If the loan was partially paid, one may not use it to borrow that part again, for that part of the lien was pardoned.

Shulchan Aruch (55:2): (If Reuven paid part of his debt to Shimon, and gave the document to Levi and said 'if I do not pay the rest by date Ploni, give the document to Shimon', Levi does so.) This is when Reuven said 'if I do not pay by date Ploni, the money I gave now was a gift.' If not, the document's lien was pardoned according to the amount he paid.

Rema: In this case, he cannot collect with it at all, only from Bnei Chorin.

SMA (2): One may collect the part that was not paid.

Rebuttal (Shach 5): Indeed, initially the Rosh connotes like this. However, there is no reason not to collect from Meshubadim the part that was never paid. We do not find in the Gemara or Poskim a decree lest he collect from Meshubadim the part that was paid. We may not invent a new decree not found in the Gemara. Some allow collecting the principal from Meshubadim with a document that sets Ribis. Some disagree, but only because it was forbidden to write the document. Therefore, it is like a predated document. Alternatively, there is more reason to decree there, for the Ribis comes from the principal. Here, the paid part does not affect the balance. (Below, we give two more reasons why there is less reason to decree here.) All Poskim and the Tur say that if one says that he paid half a document and witnesses say that he paid all of it, the lender collects half from Bnei Chorin (Bava Basra 128b). He cannot collect from Meshubadim, because the buyers say that they rely on the witnesses. If not, e.g. they agree with the borrower, he collects from Meshubadim!

Shach (2): The Rosh initially said that the lender cannot collect at all from Meshubadim, lest he collect the entire amount from Meshubadim. The borrower must pay the entire amount, so he is not concerned if it is collected from Meshubadim. Therefore we do not allow collecting from Meshubadim at all, like a predated document. The Rosh concludes that he cannot collect more than half, even from Bnei Chorin, unless the borrower stipulated that what he paid will be a gift, or did Kesivah and Mesirah. There is no concern lest the borrower let him collect more than half from Meshubadim. In many places it seems that a half-paid document may be used to collect the rest from Meshubadim. Also, one must swear to take Meshubadim. We are not concerned lest he swear falsely to collect improperly. We decree about a predated document, and some decree about a document with Ribis, for then the lender collects without swearing. The Rema cannot hold like the Rosh's initial words, for the Rosh retracted! We must say that the Rema means that he collects the unpaid part from Bnei Chorin. He should collect from Meshubadim!

SMA (2): Regarding what was paid, the Rosh requires Kesivah and Mesirah. Above, the Mordechai says that it suffices to give the document again in front of witnesses. He did not mention Kesivah. There is different, for he borrowed again that day. Why does the Darchei Moshe (ha'Aruch 54:12) say that he collects everything from Bnei Chorin? He should specify that this is only through Kesivah and Mesirah!

Rebuttal (Shach 6): The Rosh did not mean that Kesivah and Mesirah, i.e. writing 'acquire it, and its entire lien', would help. Here, part of the lien was pardoned! Also, it is predated (the second document creates a lien on the paid part, but the first document is dated earlier, unless this was all the same day). The Rosh prefers the Perush that he said that what he paid will be a gift, because the only way to acquire a document, through Kesivah and Mesirah, is not feasible here.

Gra (5 DH v'Eino): The Rema holds that Kesivah and Mesirah is not needed here like he says in Siman 66:1 (b'Sof, when from the beginning one told witnesses to write a document for the buyer and to give it to him). The Shulchan Aruch (Sof Siman 57 and Sof Siman 116) connotes that he does not collect from Bnei Chorin. However, the Rambam (Hilchos Malveh 23:1) and Shulchan Aruch (43:7) allow collecting from Bnei Chorin with a predated document. In Bava Metzia 17a, we asked why we must teach the Pesul of a pardoned lien, for in any case the document is predated. If a document in which the lien was pardoned is totally Pasul, this is not a question! Rather, it is Kosher to collect from Bnei Chorin.