Gemara

87b (Beraisa): One who reduces her Kesuvah collects without an oath.

If the Kesuvah says 1000, and he says that he paid it, and she denies this but admits that her Kesuvah is only 100, she is paid without swearing.

Question: Why does she receive (100)? Her Kesuvah is an invalid document!

Answer (Rava brei d'Rabah): She says that it was Amanah (he agreed to write a Kesuvah for 1000, and she promised him that she will claim only 100).

19a (Rav Yehudah): If he says that the document is Amanah (it was given before the loan was given), he is not believed.

Question: Who says that it is Amanah?

We cannot say that the witnesses say that it is Amanah. Obviously, they are believed only if we cannot verify their signatures without them!

Answer #1 (Rava): The borrower claims that it is Amanah. He is not believed due to Rav Huna's reason (since he admitted that the document was written properly, he is not believed with a Migo that he could have said that it was forged).

Answer #2 (Abaye): The lender says that it is Amanah. The case is, his admission hurts others (his creditors), due to R. Nasan's law (if Reuven owes to Shimon and Shimon owes to Levi, we take from Reuven to pay Levi).

Answer #3 (Rav Ashi): Really, the witnesses say that it is Amanah, and we cannot verify their signatures without them. They are not believed, due to Rav Kahana's law (one may not keep a document of Amanah in his house).

(Rav Sheshes brei d'Rav Idi): They are not believed because witnesses do not sign on fraud.

Rishonim

Rif and Rosh (9:23): We infer that had she not admitted that they agreed that her Kesuvah be only 100 (rather, the document was written for the wrong amount), the document would be worthless, and she would not collect anything.

Rosh: She is not believed with a Migo (she could have claimed that her Kesuvah really is 1000), for this is a Migo Lehotzi (to take from another). In Bava Basra (32b) a man showed a document, and said that he had a proper document and lost it, and forged this in place of it. We conclude that we leave land and money where they are.

Hagahos Ashri (9:23): It seems that this is only regarding a divorcee, and she does not have the Get or witnesses of the divorce. If there are witnesses that she was widowed or divorced, she collects without an oath, for one is not believed to say that he paid an obligation of Beis Din.

Rambam (Hilchos Ishus 16:17): One who reduces her Kesuvah is paid without an oath. This is when she shows a Kesuvah that says 1000. He says that he paid it, and she denies this but admits that her Kesuvah is only 500, and there was Amanah between them. If she says that her Kesuvah is only 500 she does not collect anything with the document, for she was Mevatel it. It is as if she admitted that it is Sheker. He swears Heses, and he is exempt.

Question (Tosfos 87b DH Oh): Since we believe one who reduces her Kesuvah (and says that there was Amanah), one who received partial payment will scheme to say that she was not paid anything, rather, she will reduce it (to avoid swearing)!

Answer (Bach EH 96:11): She prefers to say that she received partial payment, for this is common. Amanah is not common.

Poskim

Shulchan Aruch (EH 96:14): If she shows a Kesuvah that says 1000, and he says that he paid it, and she denies this but says that her Kesuvah is only 500 and there was Amanah between them, she collects without an oath.

Tur: She says that he obligated himself 1000 through a Kinyan, and trusted her to claim only 500.

Shulchan Aruch (ibid.): If she says that her Kesuvah is only 500 she does not collect anything with this document.

Beis Shmuel (28): Where people do not write Kesuvos, she collects without the document (Hagahos Ashri).

Note: Hagahos Ashri says that she collects with a Get or witnesses. It seems that the Beis Shmuel requires also that people do not write Kesuvos in the area. It is difficult to say that in such a place she collects even without a Get or witnesses, for the husband has a Migo that he never divorced her!

Rema: He swears Heses, and he is exempt.

Gra (28): One must swear Heses against any claim without a document.

Beis Yosef (CM 84 DH u'Mah she'Chosav Rabeinu v'Im): One must swear against any claim about a document.

Shulchan Aruch (CM 84:4): One who reduces his document collects without an oath. If Levi's document says 1000, and David says that he paid it, and Levi denies this but admits that David owes only 500 and there was Amanah between them, even though the document does not mention Ne'emanos, he collects without swearing, unless David demands that he swear.

SMA (11): Witnesses are not believed to say that they signed Amanah, for this is forbidden. Here, the witnesses did not know about it. The lender is believed with a Migo. It is not a Migo Lehotzi, for he has a good document.

Shulchan Aruch (ibid.): If Levi says that he was not paid, but the witnesses erred and wrote 1000, he admits that they signed Sheker, so David swears Heses, and he is exempt.

Shach (6): The Ramban, Rashba and others say that Levi is believed. Since there is a Safek about the Halachah, if Levi seized the money, he keeps it. Gedulei Terumah (p.99, 17:12) says that all agree that he is not believed when he never had a proper document. He overlooked the Ramban!