Gemara

Gitin 26b (R. Asi citing R. Yochanan): If a loan document was paid, and the document was used again for another loan between the same people for the same sum, it is void. The document's lien was pardoned when the first loan was paid.

Bava Metzi'a 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 document is invalid!

Rather, R. Yochanan teaches that the document may not be used the same day. This shows that people sometimes pay a loan the same day!

Answer (Rav Kahana): R. Yochanan teaches that we return it on the same day if the borrower admits that he did not pay. We are not concerned lest he really paid, but he denies this, in order to use it to borrow again, and save the cost of another document. No lender would agree, lest Rabanan find out and disqualify it.

Bava Basra 32b: Reuven, the Arev (guarantor) for a loan, told Shimon (the borrower) 'I paid your loan. Here is the document. Pay me back.'

Shimon said that he already paid him back. Reuven said that Shimon borrowed the money back again, and Shimon denied this.

Abaye: When one says 'believe me that he owes me money, for I could have said that my document is valid', the Halachah follows Rav Yosef. We leave the money where it is (with Shimon).

This is only when Reuven claims that Shimon borrowed again. If he says that he never accepted the money, because the coins were rubbed out or red, the document is still valid, and Shimon must pay.

Rishonim

Rif and Rosh (Bava Metzi'a 10a and 1:43): We return a validated document on the same day if the borrower admits that he did not pay. We are not concerned lest he really paid, and borrow with the document again, to save the cost of another document, and the lender will be Toref (take land that the borrower sold) improperly with a document whose lien was pardoned. The lender would not agree, for he does not gain through this, and if Rabanan find out, he will lose.

Rambam (Hilchos Malveh 14:7): If a loan document was paid, one may not use it for another loan, for its lien was pardoned and it became a (worthless) shard.

Rambam (8): If Reuven brings a validated loan document against Shimon, and Shimon says that he paid, and Reuven says that Shimon borrowed the money back again, the document is Batel. If Reuven says that the coins were not good, and he returned them until Shimon will exchange them (and pay with good coins), the document is valid and its lien is intact.

Magid Mishneh, citing R. Yonah: Even if the coins are accepted grudgingly, Reuven can say that had he known, he would not have accepted them.

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 and it would not be predated, 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): If a lender claims with a validated document, and the borrower says that he paid, and the lender admits but says that he had a proper document and lost it, he does not collect, for he admits that the document he holds was never given for collection. Here, the document was given for collection! This is like a predated document. The Rif and Rashi say that one may collect with it from Bnei Chorin. Those who argue make a fine not to allow collecting with it from Bnei Chorin, lest one collect Meshubadim from the date written. It seems that they would not fine here. The borrower is not believed to say that he paid, for one does not pay and leave a signed document with the lender.

Defense (Shach 2): It is clear from Bava Basra 32b and the Rashbam (DH Ki) that if one says that he lent again with the same document, it is Batel and the borrower is believed. The Rambam, Semag, Sefer ha'Terumos, Rivash, Tur and Shulchan Aruch (57:2) agree. The Bach says that one cannot collect with a Kesav Yado (a document in the borrower's handwriting) if the lien was pardoned. Also a Kesav Yado is used only to collect Bnei Chorin.

Ir Shushan (ibid.): If one has a Kesav Yado with Ne'emanos (the borrower is not believed to say that he paid), which can be used only to collect from Bnei Chorin, one may borrow with it again (after paying the first loan).

Shach: This is correct. Giving back the Kesav Yado is like signing again, just like giving a signed document helps as if the witnesses signed again.

SMA (2): I agree, but only if he borrows on the same day. (if not, the lender could collect improperly.) One cannot take Meshubadim with a Kesav Yado, but we hold that an earlier Milveh Al Peh has priority (for Bnei Chorin) over a later loan document. All agree that an earlier Milveh Al Peh has priority over a later Milveh Al Peh.

Rebuttal (Shach 2): There is priority only if there are witnesses about the earlier loan (CM 104:13). If so, it need not be on the same day!

Rema: Some say that 'the lien was pardoned' refers to the first Kinyan. Therefore, if he did another Kinyan, he can borrow with it again. This is even if he was not Makneh to him again, just he gave it to him again in front of witnesses, according to the opinion that Edei Mesirah are primary for monetary documents.

Source (Mordechai Kesuvos 218, cited in Beis Yosef DH v'Chosav): The Ri allows borrowing with it again if Edei Mesirah saw it given again, for the primary lien comes through giving it over. Therefore, it makes a new lien on the borrower's property, since it was signed this day. He holds that Edei Mesirah are primary for monetary documents.

Shulchan Aruch (57:2): If Shimon says that he paid the loan, and Reuven says that he lent the money to him again, Reuven admits that the lien was pardoned. If Reuven says that he returned the coins so that Shimon will exchange them, since they are accepted only grudgingly, the document is not Batel.

Shach (8): We do not believe that Reuven lent the money again Migo (since) he could say that Shimon never paid. We do not make people pay due to a Migo. The opinion that disagrees must say that Reuven admitted that he was paid before he validated the document. Afterwards it is a shard, even if he validates it. However, we follow Rav Yosef (Bava Basra 32b) only due to a Safek, so if the lender seized the money, he keeps it.