Gemara

Question (Rav Amram): If Reuven has a document saying that he deposited with Shimon, and Shimon claims 'I returned it to you', what is the law?

Do we say that since Shimon would be believed to claim 'it was lost through Ones', he is believed to say 'I returned it'?

Or, since Reuven still has the document, this proves that he did not return it?

Answer (Rav Chisda): Shimon is believed.

Question (Rav Amram): If he returned it, why does Reuven still have the document?

Answer (Rav Chisda): (Shimon returned it even though Reuven said that he lost the document. He was not afraid lest Reuven later claim with the document, for) Shimon could say that it was lost through Ones, and the document would not disprove this!

Question (Rav Amram): Shimon would have to swear (that it was lost through Ones)!

Answer (Rav Chisda): I meant that Shimon is believed if he swears (that he returned it).

Shevuos 45b (Shmuel): If there are no witnesses that a worker was hired, the employer is believed, Migo he could say that he never hired him.

Objection (Rava): If such a Migo exempts from swearing, when did the Torah obligate Shomrim to swear?! Since a Shomer is believed (without swearing) to say that he never received the deposit, he is believed to say that it was lost through Ones!

Answer (Rami bar Chama): The case is, the depositor has a document that the Shomer received the deposit.

Inference: He and Rava hold that if a document was written, he must return the deposit with witnesses (or take back the document).

Rishonim

Rif (Bava Metzia 25b): If Reuven has a document saying that he deposited with Shimon, and Shimon claims that he returned it, since Shimon would be believed to claim that it was lost through Ones, he is believed to say that he returned it. In either case Shimon must take a severe oath while holding a (Kodesh) item.

Rambam (Hilchos Sechirus 2:12): If Reuven has a document saying that he deposited with Shimon, and Shimon claims 'I returned it to you', Shimon is believed, but he must swear while holding an item. Since a Shomer Chinam would be believed to claim 'it was lost or stolen', and a borrower could say 'it died while working', he is believed to say 'I returned it.' Just like if he claimed that Ones occurred he would need to swear mid'Oraisa while holding an item, if he claims that he returned it, he must swear like a mid'Oraisa oath, since the claimant has a document. This is when Shimon could have exempted himself without a proof of his claim. However, if he would have needed a proof for his claim, like we will explain, he is not believed to say that he returned it. Rather, Reuven swears while holding an item that he did not return it, and Shimon pays.

Magid Mishneh: The Gemara did not distinguish between whether or not witnesses are found, but it is clear, for there is no Migo.

Ra'avad: This is if Shimon asks him to swear. L'Chatchilah, we tell Shimon to pay.

Magid Mishneh: The Ra'avad equates this to a borrower who claims that he paid. We tell him to pay, and if he asks the lender to swear, the lender must swear, like the Rambam says in Perek 14 of Hilchos Malveh. This is correct. The Rambam did not elaborate here.

Rambam (ibid): A Shomer against a document is the only case in which one swears because he could have claimed differently and sworn while holding an item. All others who swear due to a Migo swear only Heses.

Rosh (Shevuos 7:2): R. Tam says that the Halachah does not follow Rami bar Chama and Rava. Rather, we hold like Rav Chisda, that even if a document was written, one may return without witnesses. He is believed through an oath to say that he returned it. According to R. Tam, we can explain here that one must return with witnesses in order to be exempt from swearing. Also R. Yosef ha'Levi says that this (Rami bar Chama's opinion) is not the Halachah.

Piplulei Charifta (6): The latter words of R. Tam contradict his initial words! Tosfos cites only the latter words in the name of R. Tam.

Tosfos (45b DH bi'Shtar): When there is a document, he should be exempt through a Migo that Ones occurred, like in Bava Basra! R. Tam says that he must return with witnesses to exempt himself from swearing. The Riva says that here there is no reason to leave the document with the depositor. Regarding an investment, the borrower can say that the lender kept the document until the borrower will pay the cost of the document.

Gra (CM 296:4): The Ge'onim say that the Halachah does not follow the Sugya in Shevuos.

Poskim

Shulchan Aruch (CM 296:2): If Shimon says 'you did not deposit with me, lend or rent to me', or 'you deposited with me, but I returned it', he swears Heses and he is exempt. Even if Reuven has a document saying that he deposited with Shimon, if Shimon claims 'I returned it to you', he is believed, but he must swear while holding an item. Since he would be believed to claim that it was stolen, i.e. if he is a Shomer Chinam, or that Ones occurred if he is a Shomer Sachar, or that it died while working if he is a borrower, he is believed to say that he returned it. Just like if he made one of the former claims he would need to swear mid'Oraisa while holding an item, if he claims that he returned it, he must swear like a mid'Oraisa oath.

SMA (3): Even though a Migo does not exempt from a Shevu'ah mid'Oraisa, Shevu'as ha'Shomrim does not apply here. However, if Reuven has a document, Shimon must swear mid'Oraisa, just like he would swear mid'Oraisa that there was Ones to exempt himself against a document. Shimon was not particular to get back the document, for he knew that he can exempt himself through a Migo.

Bach (2): The Tur, Rif and Rambam say that he must swear mid'Oraisa. This is unlike Tosfos (70b DH Sof), who say that the oath is mid'Rabanan, but it was enacted before Heses. Tosfos says so only according to the opinion that Shevu'ah ha'Shomrim is mid'Oraisa only when there is partial admission. We hold that it is mid'Oraisa even without partial admission.

Shach (3): The Bach understands that they (and Ir Shushan) literally mean a Shevu'ah mid'Oraisa. I say that it is only like mid'Oraisa, i.e. while holding an item. Surely the Torah obligated a Shevu'ah only when he claims Ones, lest everyone claim this, for the depositor does not know whether or not he is lying. When he claims 'I returned it to you', the depositor knows whether he lies, and there is no Shevu'ah mid'Oraisa. Tosfos and the RIf agree. R. Shimshon in Sefer ha'Krisos explicitly says that it is mid'Rabanan. The Ba'al ha'Ma'or (37a) and Tosfos (34a DH Mechuyav) agree; therefore, we do not obligate him if he cannot swear. Also, a mid'Rabanan oath differs regarding taking his property, reversing the oath and one who is suspected to swear falsely.

Shach (4): The Rosh (Bava Metzia 1:3) says that if one witness supports Shimon, he is exempt from swearing. This contradicts the Rema (87:6)/

Shulchan Aruch (3): This is when Shimon could have exempted himself without a proof of his claim. However, if he would have needed a proof for his claim, e.g. in a place where witnesses are found, he is not believed to say that he returned it. Rather, Reuven swears while holding an item that he did not return it, and Shimon pays. This is if Shimon asks him to swear. L'Chatchilah, we tell Shimon to pay.

SMA (4): The Torah says "if no one sees, he will swear." If others see, he is not believed to swear. The Rambam and Tur connote that in a place where witnesses are found, surely witnesses would have seen the Ones. This implies that Stam (if we do not know that witnesses are found) he need not bring a proof. The Shulchan Aruch holds like the Ramah, that Stam he must bring a proof.