Gemara

(Rabah): Rav holds that if one tries to simultaneously do two things, one of which cannot take effect after the other, neither takes effect.

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 where 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.

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]. Rav Nachman suggested that she pardon her mother's Kesuvah to her father, [and she will inherit her father]. She did so.

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

89a - Question: Even if she has no Nichsei Melug to pay, 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!

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 Levi. Levi claims that Shimon agreed not to press him for payment until Levi wants. Can Reuven collect from Levi due to Shibud d'R. Noson?

Answer (Rosh 69:3): If the document explicitly says that Levi need not pay until he wants, Reuven has no more lien against Levi than Shimon did. If the document does not say so, R. Yosi gets Shimon's right to collect whenever he wants. Reuven merited Shimon's Shibud against Levi. Shimon cannot pardon it or extend the time. He is unlike one who sold a loan document, who can pardon it, for the sale is only mid'Rabanan.

Contradiction: The Rosh in Kesuvos holds that Shimon (who owes and is owed) can pardon Levi!

Answer #1 (Bach CM 86 DH She'elah): The Rosh holds that Shimon can pardon before it is known (Taz 86:5 - to Beis Din) that he has no other property. He cannot pardon after it is known, for then Shibud d'R. Noson applies.

Rebuttal (and Answer #2 - 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.)

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.

SMA (65): If the seller previously owed to the buyer, the buyer had a lien on the document due to R. Noson even without the sale, so the seller cannot pardon it (CM 86:5). The Rema (Sa'if 1) said that [if the seller took the money and retracted from the sale, and he cannot return the money,] the buyer collects from the borrower, i.e. what he paid for the document. Here is different, for the seller pardoned the borrower, even if he cannot pay.

Shach (89): If he did not owe him from elsewhere, he can pardon, even if he cannot pay for the loss he causes. The Beis Yosef says that Teshuvas ha'Rosh (68:3) connotes otherwise. This is wrong, and the Poskim say unlike this. The Rosh discussed a woman who owed others. She cannot pardon her Kesuvah [for her creditors have a Torah lien due to R. Noson. We cannot learn to one who sold a debt, which is only mid'Rabanan.] The Bach (31) rejected the Beis Yosef. The Rosh (Kesuvos 9:16) and Tur said that if the seller does not want to swear, the buyer loses. The Beis Yosef explains that this is if the seller has no money. The Rosh (9:28) explained differently! The seller must swear [that he was not paid, before the buyer can collect] because the seller is believed to say that he paid, Migo he could pardon.

Shach (89): Ra'avan says that he is not believed to say that the document was Emanah [if he does not have property], Migo he could pardon it, because one may pardon only when he can pay for the loss. The Gemara and Poskim say unlike this. Tosfos and the other Rishonim gave other answers why he cannot say that it is Emanah. Bava Kama 89a proves that one can pardon even if he cannot pay. The Ran says that a Kesuvah is different, for it is not destined to be collected in his lifetime. I wrote that this is unreasonable. And even according to the Ran, according to Ra'avan, since the buyer loses, why can she 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.

Ketzos ha'Choshen (34): Why should it matter whether or not the seller can pay? He can pardon, and he owes for this! Why should the borrower lose the pardon he received, just because the pardoner cannot pay?! Perhaps the Rosh holds that when he pardons, he must pay due to Dina d'Garmi (directly causing a loss), and he cannot pay. He cannot pardon due to Shibud d'R. Noson. The buyer gets a lien against the seller and borrower. Even though there was no debt before the pardon, both are obligated to pay the buyer, since the pardon and Shibud d'R. Noson come simultaneously. Therefore, the pardon does not help. Whenever two matters do not work one after the other, also at once they do not work (Eruvin 50a). The Ran (Kesuvos 31b) says similarly about one who vowed that his wife may not benefit from him until 30 days.

Note: The case is, she was used to small things in her father's house, but not with her husband. She could manage without them only until he vowed. He becomes obligated at the moment he vows. Why is his vow valid? This is unlike a vow during Eirusin [before he was obligated]! The Ran (DH d'Havi) answers that a vow uproots the lien. Normally, Chachamim made her lien stronger, but not in this case.

Ketzos ha'Choshen: This explains why in Bava Kama 89a we say that she will surely pardon to her husband, even though she cannot pay. Shibud d'R. Noson does not apply to a Kesuvah, like the Ran said (Kesuvos 6a), for perhaps it will never be collected. Therefore, the pardon works.

Rema (ibid.): Some disagree.

Gra (110): Kesuvos 86a proves that Shimon can pardon.

Shulchan Aruch (86:5): [If Levi owes Shimon and Shimon owes Reuven,] 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): Shibud d'R. Noson applies only when the middleman cannot pay. Bava Kama 89a discusses a woman without Nichsei Melug [yet she can pardon]! The Rema (66:26) answered that the seller can pardon if he did not already owe the buyer. If he is liable only due to the sale (he sold a loan document and pardoned it), it is not a sale, and the money was a gift. Bava Basra 147b proves this. It says that selling a loan document works only mid'Rabanan. It connotes that this is even if the seller has no other property, for it compares it to a Shechiv Mera. The borrower should owe the buyer mid'Oraisa, due to Shibud d'R. Noson! (Rather, he does not owe, for the money paid was a gift.)