Gemara

(Beraisa): If one found a receipt (for payment of Leah's Kesuvah), if she admits (that she was paid), we return it to her ex-husband.

Question: Perhaps he paid after the date written, and she sold her Kesuvah in between. The receipt 'proves' that he paid the Kesuvah before she sold it!

Answer #1 (Rava): This shows that Shmuel is correct.

(Shmuel): One who sells a loan document can pardon the debt. (Since Leah can pardon the Kesuvah anyway, returning it does not hurt the buyer.)

Rejection (and Answer #2 - Abaye): This is not a proof for Shmuel. The case is, she is holding her Kesuvah (so we are not concerned lest she sold it).

Gitin 13b (Shmuel): If Reuven says to Shimon 'you borrowed money from me. Give it to Levi', b'Ma'amad Sheloshtan (when all three are present), Shimon owes the money to Levi.

Kesuvos 86a (Shmuel): One who sells a loan document can pardon the debt.

(Rav Huna brei d'Rav Yehoshua): If the buyer is clever, he will offer money to the borrower to write a loan document obligating himself to the buyer.

Kidushin 47b (Beraisa #1 - R. Meir): If one was Mekadesh a woman with a loan document, or gave her rights to collect a debt owed to him, she is Mekudeshes;

Chachamim say, she is not Mekudeshes.

Regarding a loan document, they argue about Shmuel's law, that if a lender sold his loan document, and then pardoned the debt, it is cancelled. Alternatively, all hold like Shmuel. They argue about whether a woman resolves to accept the document, or if not, due to fear lest he pardon the debt.

Regarding a Milveh Al Peh (a loan without a document), they argue about whether Ma'amad Sheloshtan applies only to deposits, or even to loans.

Bava Kama 89a (Mishnah): If a married woman damaged, she is exempt (she cannot pay). We do not say that she must sell the Tovas Hana'ah of her Kesuvah (rights to collect, if she will be widowed or divorced), due to Shmuel's law. (Surely, she will pardon the Kesuvah to please her husband!)

Question: She should sell the Tovas Hana'ah of her Kesuvah to the damagee. Even if she will pardon the loan, it is no worse for him than collecting nothing

Answer: Surely, she will pardon the Kesuvah to please her husband. We do not exert Beis Din to arrange a sale that will not help the damagee.

Bava Basra 147b: Rav Nachman holds that Matnas Shechiv Mera takes effect only in those cases when a gift of a healthy person takes effect.

Question: Rav Nachman taught that if a Shechiv Mera said 'the debt owed to me should be to Ploni', it takes effect, even though a healthy person cannot do this!

Answer #1 (Rav Papa): Matnas Shechiv Mera makes Ploni the heir (and heirs acquire debts), therefore he acquires the debt.

Answer #2 (Rav Acha brei d'Rav Ika): A healthy person can transfer a debt through Ma'amad Sheloshtan.

Rishonim

Rambam (Hilchos Mechirah 6:8): If Reuven, Shimon and Levi were present, and Reuven told Shimon 'you have money of mine, a deposit or loan. Give it to Levi', Levi acquires it. None of them can retract.

Rambam (Hilchos Ishus 5:17): If David owed money to Reuven, and Reuven told Leah 'you are Mekudeshes to me with the money that David owes me' b'Ma'amad Sheloshtan, she is Mekudeshes.

Magid Mishneh: She resolves to be Mekudeshes because he cannot pardon the debt.

Rosh (Gitin 1:17): Normally, if one sold a debt and later pardoned it, the buyer cannot collect. If he gave through Ma'amad Sheloshtan, he cannot pardon it. We can prove this from Kidushin 47b. The Gemara did not conclude that all agree that Ma'amad Sheloshtan applies even to loans, and they argue about whether or not she fears lest he pardon the debt.

Rebuttal (Ramban Bava Basra 144a DH va'Ani): According to the answer that they argue about Shmuel's law regarding a loan document, the same applies to a Milveh Al Peh!

Question #1: Bava Metzia 19b proves that one can pardon even if he gave through Ma'amad Sheloshtan. If not, we should be concerned lest she sold her Kesuvah through Ma'amad Sheloshtan!

Answer #1 (Ramban DH Maneh): When her husband was in the Ma'amad Sheloshtan, he admitted that he did not yet pay the Kesuvah, so a receipt saying that he paid it beforehand would not exempt him.

Tosfos (Gitin 13b DH Tenehu): If there are no witnesses to the sale, in any case she could deny that she ever sold it. If witnesses say even that the husband was silent, this is like admission that he did not pay it.

Question #2: In Bava Kama 89a, we say that it is futile to make a woman sell her Kesuvah to pay a damagee, for surely, she will pardon the Kesuvah. We do not force her to sell through Ma'amad Sheloshtan!

Answer (and Answer #2 to Question #1 - R. Tam, cited in Ramban and Rosh Bava Metzia 1:48): Ma'amad Sheloshtan works only for something that can be collected immediately. Perhaps the Kesuvah will never be collected!

Poskim

Shulchan Aruch (CM 66:29): If one gave or sold a loan document through Ma'amad Sheloshtan, he cannot pardon it.

Beis Yosef (66 DH v'Chen): The Nimukei Yosef (Bava Basra 67a DH Omar) says that most Meforshim say that one who sold a loan document, even through Ma'amad Sheloshtan, can pardon it. This is wrong. Most say that in this case he cannot pardon it, including Tosfos, the Rosh and Ran.

Rebuttal (Shach 126:10): Perhaps the Nimukei Yosef saw more Meforshim who disagree with Tosfos, e.g. the Ra'avad, Rashba, Ramah, Ba'al ha'Ma'or, and Ri mi'Gash! Also the Magid Mishneh leans to say that he can pardon.

Shach (97): The Rishonim say that Ma'amad Sheloshtan works for loans with a document. It transfers the debt, but not the document. The giver can pardon even if he also gave the document through another document.

Rebuttal (Nesivos ha'Mishpat Urim 39): This is unreasonable. The giver can pardon because he retains Shibud ha'Guf (the borrower owes him). Once it lapses there is no Shibud of property. Ma'amad Sheloshtan transfers Shibud ha'Guf. It leaves over only the document, which is Shibud of property. Since he transferred the document through another document, he cannot pardon it!

SMA (71): He cannot pardon it because it is as if a (new) loan was made and the borrower obligated himself to the recipient.

Tumim (54): Bava Basra 148a proves that one who gave through Ma'amad Sheloshtan cannot pardon! Rav Nachman holds that Matnas Shechiv Mera helps only when a gift of a healthy person takes effect, and he taught that a Shechiv Mera can transfer a debt. Rav Acha answered that also a healthy person can transfer a debt through Ma'amad Sheloshtan. We must say that it is in the same way, i.e. the giver cannot pardon it. (One cannot pardon a gift through Matnas Shechiv Mera, for it is like an inheritance.)