Gemara

(Mishnah): If a woman shows a Get without a Kesuvah document, she collects a Kesuvah. If she shows a Kesuvah without a Get and says that she lost her Get, and he says that he (paid it, tore her Get, and) lost his receipt, he need not pay.

R. Shimon ben Gamliel says, after there were decrees not to keep Mitzvos (we destroy a Get after it is given), a woman collects her Kesuvah without a Get.

Inference: This shows that we (force a borrower to pay when the creditor says that he lost his document, and) write a receipt. If not, we should be concerned lest she collect with her Get, and show her Kesuvah (after her ex-husband dies, claim that she was widowed) and collect again!

Rejection (Rav Anan citing Shmuel): In a place where Kesuvos are not written, if he claims that he wrote a Kesuvah, he must bring proof (or pay). Where Kesuvos are written, if she claims that he did not write one, she must prove this (to collect).

(Rav): Whether or not Kesuvos are written or not in the place, if she shows (only) a Get, she receives the basic Kesuvah (100 or 200). If she shows (only) a Kesuvah, she receives any Tosefes.

Question (Mishnah): If she shows a Kesuvah without a Get and says that she lost her Get, and he says that he lost his receipt, he need not pay.

Shmuel establishes this to be in a place where Kesuvos are not written. He claims that previously she showed her Get, and he paid because he could not prove that he had written a Kesuvah. Therefore, he does not pay now;

According to Rav, he should pay the Tosefes!

Answer (Rav Yosef): The case is, there are no witnesses who saw the divorce. Migo (since) he would be believed to say that he never divorced her, he is believed to say that he divorced her and paid the Kesuvah.

Question (against Rav): You hold that the basic Kesuvah is collected only with a Get. How does a widow from Eirusin collect? If we rely on witnesses that her husband died, we should be concerned lest she was divorced and will collect again with her Get!

Answer: When there is no other solution, Rav agrees that we write a receipt.

If not, when she collects with witnesses of death, we should be concerned lest she collect again with other witnesses in a different Beis Din!

Bava Metzi'a 17a (R. Yochanan): A claim against acts of Beis Din is ignored.

This is because it is as if a document was written for the debt.

Question (R. Chiya bar Aba): We know this from the Mishnah that says that a woman collects her Kesuvah through showing her Get, even without a Kesuvah!

R. Yochanan: You understood the Mishnah due to my teaching!

Question (Abaye): Perhaps the Mishnah discusses where they do not write Kesuvos, so the Get is in place of the Kesuvah, but where they write Kesuvos, she does not collect without a Kesuvah!

Retraction (Abaye): If where they write Kesuvos she does not collect without a Kesuvah, a widow from Kidushin would collect only with witnesses of death. If so, it was futile to enact a Kesuvah for widows from Kidushin. His heirs could claim that she was already paid!

Rishonim

Rif and Rosh (9:30): Shmuel establishes the Mishnah to be in a place where Kesuvos are not written. The Halachah follows him.

Rif (Bava Metzi'a 10a): R. Yochanan taught that a claim against acts of Beis Din is ignored. It as if a document was written.

Rebuttal (Ba'al ha'Ma'or): R. Yochanan understands that the Mishnah discusses one who admits that he owes, but he refuses to pay until the other party produces the document. R. Yehudah holds that (normally) we do not write receipts, but he agrees regarding acts of Beis Din. We do not need this ruling, for we write receipts for all debts! Alternatively, the Chidush is that we write a receipt for a Kesuvah, even though we cannot apply "a borrower is the slave of the lender." He claims that he lost his receipt, i.e. the Get, on which the receipt was written. Abaye wanted to explain the Mishnah like the Sugya in Kesuvos (that he claims that he paid). He retracted to explain like R. Yochanan. How can the Rif rule here like R. Yochanan, and there like Shmuel?! In a Teshuvah he wrote that the Mishnah discusses only Ikar Kesuvah, and he cannot claim against it. This is not clear at all.

Defense (Rosh Bava Metzi'a 1:41): We should follow R. Yochanan against Shmuel, especially since Abaye, who is Basra, supports him! The Ramban says that Shmuel does not argue with R. Yochanan. In a place where they write Kesuvos, her claim, that she lost it, is feeble. It seems that it was paid and torn. At first, Abaye thought that the Get is like the Kesuvah. He asked how our Mishnah supports R. Yochanan, who says that one cannot claim against Ma'asei Beis Din even where they write. Perhaps he cannot claim due to the Get! Later, Abaye realized that we cannot say that the Get is like the Kesuvah. The Mishnah and R. Yochanan discuss only where they do not write. If not, he can claim against Ma'asei Beis Din, because her claim is feeble; it seems that the debt was paid. The enactment was not futile; at the time of divorce, she can warn him to pay her only in front of Ploni and Almoni. The Chidush of Ma'asei Beis Din is where they do not write Kesuvos, and witnesses say that he wrote. He is not believed to say that he paid. Since the custom is not to write, he caused himself the loss by writing it. It does not change the law. Since we need not say that Rav and Shmuel argue with R. Yochanan, this is most reasonable.

Ran (DH ul'Inyan): All agree that he is believed to say that he paid the Tosefes. The Rif holds that he is believed even about Ikar Kesuvah where it is written, and Tosefes is written everywhere. Even those who say that he is not believed where it is written say so only about Ikar Kesuvah, which is Tnai Beis Din. Tosefes is not a Tnai Beis Din.

Rambam (Hilchos Ishus 16:27): If (she shows a Kesuvah without a Get and says that she lost her Get, and) he says that he paid the entire Kesuvah and lost his receipt, since he could have said that he never divorced her to exempt himself from Tosefes, he is believed. He makes her swear holding an item (of Kedushah, e.g. a Sefer), and pays Ikar Kesuvah. He swears Heses that he paid the Tosefes.

Rambam (28): If she shows a Get without a Kesuvah, if it is normal in this area not to write a Kesuvah, she collects Ikar Kesuvah through the Get. If it is normal in this area to write a Kesuvah, she does not collect even Ikar until she shows her Kesuvah. Her husband swears Heses against her claim, and he is exempt.

Nimukei Yosef (Bava Metzi'a 4b DH v'Chen): If a man says that during the marriage he gave her Tzrari (money, to ensure that she will collect if she is widowed), this is no claim, for the Kesuvah is not collectable in his lifetime. He pays, and cannot make her swear. He can impose a Cherem Stam (on swindlers).

Poskim

Shulchan Aruch (EH 100:12): If she shows a Get without a Kesuvah, if it is normal in this area not to write a Kesuvah, she collects Ikar Kesuvah through the Get. If it is normal in this area to write a Kesuvah, she does not collect even Ikar until she shows her Kesuvah. Her husband swears Heses against her claim, and he is exempt.

Ran (Kesuvos 49a DH Gemara, brought in Beis Yosef DH Hotzi'ah and Chelkas Mechokek 42): She collects with a Get, but not with witnesses of divorce. He can say that he paid her and tore the Get!

Beis Shmuel (41): This is according to the Rif, who says that she is not believed when her claim is weak Tosfos says that she collects even where they write Kesuvos and her claim is weak. Similarly, she collects even when she does not have her Get! The Tur holds like the Rif, yet he says that she collects with a Kesuvah and no Get! We can say that since she has her Kesuvah, there is no weakness in her claim.