Gemara

19b - Question: Why does a Beraisa permit returning a receipt for payment of a Kesuvah (to the husband) if she admits? Perhaps it was given after the date written, after she sold her Kesuvah after the date but before she was paid. The receipt is a false 'proof' that the Kesuvah was paid before she sold it!

Answer (Rava): This is a proof for Shmuel, that if one sold a loan document, and then pardoned the debt, the debt is cancelled. (Since she can pardon the Kesuvah anyway, returning the document does not hurt the buyer.)

Kesuvos 19a (Rav Yehudah): If he says that the document is Amanah (the loan was never given), he is not believed.

(Abaye): The lender says that it is Amanah. It is a case in which his admission hurts others (his creditors), like R. Noson's law:

Beraisa (R. Noson): "He will give to the one he sinned against" - if Reuven owes Shimon, and Shimon owes Levi, we take from Reuven to pay Levi.

81b (Beraisa): Reuven owed money to his brother Levi. Levi died and his wife fell to Yibum. Reuven cannot say 'since I (will do Yibum and) inherit, I will keep the money.' Rather, we take the money and buy land with it, and he eats the Peros.

He cannot tell her 'you have no claim against me.' This is like R. Noson.

85b: A relative of Rav Nachman sold her Kesuvah. She was divorced, and then died. The buyer came to collect from her daughter (who inherited her, and claimed her mother's Kesuvah from her father). Rav Nachman suggested that she pardon her mother's Kesuvah to her father, (and she will inherit that money from her father). The daughter heard the counsel, and did so.

Bava Kama 87a (Mishnah): If a (married) woman damaged someone, she is exempt now, but pays later.

89a - Question: Granted, we can say that she has no Nichsei Melug to pay. However, she should sell the Tovas Hana'ah of the Kesuvah to pay the damage!

Answer: She need not sell it, for Shmuel taught that one who sold a loan document can pardon it. (Surely, she will pardon it to please her husband!)

Gitin 40b (Mishnah): If Shimon lent to Reuven money, and Reuven made his slave Tavi collateral (with a guarantee that the loan may be collected from Tavi's value) and he freed him, letter of the law, Tavi owes nothing to his master.

(Rav): Reuven freed him. Letter of the law, Tavi owes nothing to Shimon, who should have received him. Rava taught that freeing a slave uproots a lien. Chachamim exempt one who damages property Meshubad to someone else.

Rishonim

Rosh (and Tosfos, Kesuvos 2:9 and 19a DH u'Chegon): Shimon is not believed to say that the document was Amanah, Migo (since) he could pardon the debt. This is not a Migo, for perhaps he plans to collect. He assumes that the borrower does not want to steal, and will pay later.

Tosfos (ibid.): Alternatively, if the sale of a loan document is only mid'Rabanan, perhaps one cannot pardon when Shibud d'R. Noson applies, for it is mid'Oraisa.

Beis Yosef (CM 86 DH v'Da): The Rosh only brought Tosfos' first answer. This implies that he holds that one can pardon when Shibud d'R. Noson applies (unlike the Teshuvah below).

Question: Reuven had a loan document against Shimon, and Shimon had a loan document against his brother Levi. Reuven wanted to collect from Levi due to Shibud d'R. Noson. Levi claims that his brother kindly agreed not to press him for payment until Levi wants. Beis Din senses that this is true. Is Levi's claim valid?

Answer (Rosh 69:3): If the explicitly says that Levi need not pay until he wants, Reuven has no more Shibud against Levi than Shimon did. We do not say 'as long as Shimon owes Reuven, he is his slave, and may not do Chesed with his master's money.' Shimon may give gifts to anyone he wants. Levi is Meshubad to Reuven only due to the loan. Once the loan was given, Reuven merited Shimon's Shibud against Levi, and Shimon cannot pardon it or extend the time. He is not believed to say that the document is Amanah. He has no Migo that he could pardon it. He is unlike one who sold a loan document, who can pardon it, for the sale is only mid'Rabanan. If the document does not say that he cannot force Levi, even if it is known that he would not have forced him, Shimon could collect whenever he wants, and Reuven has that right.

Contradiction: The Rosh in Kesuvos holds that Shimon can pardon Levi!

Answer #1 (Bach CM 86 DH She'elah): The Rosh holds that Shimon can pardon before it is known that he has no other property. He cannot pardon after it is known, for then Shibud d'R. Noson applies.

Rebuttal (Shach 47:7): The Rosh in Kesuvos discusses when Shibud d'R. Noson applies. In the Teshuvah he discusses the same case as in Kesuvos! (Rather, the Rosh retracted to say that he cannot pardon.)

Answer #2 (Taz 86:5): The Rosh (Teshuvah 90:1, cited in Beis Yosef DH Kosav) says that Shibud d'R. Noson starts only when it becomes clear to Beis Din that Shimon owes him. Before this, he can pardon.

Poskim

Rema (CM 66:26): Some say that if Shimon sold a loan document, he can pardon it only if he did not owe the buyer, or if Shimon has other property to pay the debt. If Shimon owed the buyer and had no other property, the borrower already is Meshubad to the buyer, so Shimon cannot pardon.

Gra (108): We learn from Bava Kama 89a (that one who does not owe can pardon, even if he has no other property). If a woman without money sells her Kesuvah, she can pardon it (for once she sells it, she does not owe). Also, Rav Nachman counseled a girl to pardon her mother's Kesuvah so the buyer would not collect. If Reuven owed to his brother Levi, and Levi died and his wife fell to Yibum, we take the money from Reuven, buy land with it, and he eats the Peros. Reuven will (do Yibum and) inherit Levi, but he cannot pardon the debt to himself, for he is Meshubad to her due to R. Noson's law.

Rema (ibid.): Some disagree.

Gra (110): Tosfos (Kesuvos 19a) rejected the above proofs. (Reuven cannot uproot the Yevamah's rights, for he inherits only due to Yibum with her). Gitin 40b supports Tosfos. Sefer ha'Terumos says that freedom is like pardon (and it works). Really, this proves the opposite! (Only) freedom works, for it is like Kedushas ha'Guf. (The slave becomes a Yisrael, and cannot revert to being a slave. Hekdesh Kedushas Damim does not uproot a lien - Tosfos 40b DH Hekdesh, based on the Yerushalmi.) Kesuvos 86a proves that Shimon can pardon. The Ramban brings a proof from Bava Metzia 20a. We return a receipt, even though perhaps she borrowed from others (and the receipt will exempt her husband from Shibud d'R. Noson). We can reject this; perhaps we return when she has Nichsei Melug.

Shulchan Aruch (CM 86:5): Shimon cannot pardon Levi or extend the time he has to pay, for his Shibud on Levi was uprooted. Levi is now Meshubad to Reuven.

Gra (16): Bava Kama 89a discusses a woman without Nichsei Melug, yet she can pardon! The Rema said that one who did not owe can pardon.