Gemara

(Mar Zutra): Chachamim made three enactments without reason (how they work). One of them is Rav's law:

(Rav): If Reuven says to Shimon 'you have money of mine. Give it to Levi', and all three are present, Levi acquires the money.

Gitin 13b (Ameimar): It is as if Shimon said when he borrowed the money 'I am indebted to you, or anyone who will come in your stead.'

Objection (Rav Ashi): If so, one should not be able to give the loan to someone who was not born at the time the loan was given!

(Mar Zutra): Rather, this is an enactment without reason.

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 (a lender who sold his loan document can pardon it). Alternatively, all hold like Shmuel. Chachamim holds that she lacks resolve to accept Kidushin, due to fear lest he pardon.

Regarding a Milveh Al Peh (a loan without a document), they argue about Ma'amad Sheloshtan. Chachamim hold that it applies only to deposits, but not to loans. R. Meir holds that it applies 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 (collection rights) of her Kesuvah, due to Shmuel's law. Surely, she will pardon the Kesuvah to please her husband!

Bava Metzi'a 19b (Beraisa): If one found a receipt for payment of a Kesuvah, if she admits that she was paid, he returns it to her husband.

(Rava): We are not concerned lest it was given after the date written, and she sold her Kesuvah in between (and the receipt 'proves' that he paid the Kesuvah beforehand), for in any case she could pardon the Kesuvah, like Shmuel taught.

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).

Rishonim

Rosh (Gitin 1:17) and Tosfos (13b DH Tenu): If one gave a debt through Ma'amad Sheloshtan, he cannot pardon it. The Gemara (Kidushin 47b) 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!

Poskim

Shulchan Aruch (CM 66:19): If Reuven had a loan document against Levi, and said 'give the money you owe to me to Shimon', in the presence of all of them, Shimon acquires, even if he did not get the document. Buyers (who bought Levi's property) have a Shibud to pay Shimon, like they were obligated to pay (Reuven). Shimon cannot force Reuven to give to him the document. Rather, Shimon claims from Levi, and if Levi denies it or Shimon needs to collect from sold property, Beis Din forces Reuven to produce the document, and we rule according to it. If Levi paid Shimon, we take the document from Reuven and give it to Levi.

SMA (49): This helps even for a Milveh Al Peh. The Tur and Shulchan Aruch teach that even though Shimon acquires the debt, he cannot force Reuven to give to him the document. This is because there are two Kinyanim on documents: the document itself, and the lien. Neither is acquired through transfer of the other.

Shach (97): Throughout Shas we say that the seller can pardon, and do not say that Ma'amad Sheloshtan is different. This is difficult for Tosfos. Rav Huna said that if one who bought a document is clever, he will pay the borrower to write a loan document obligating himself to the buyer, so that the seller will be unable to pardon it (Kesuvos 86a). He did not counsel to buy through Ma'amad Sheloshtan, which would give a better lien, from when he bought the loan, before the latter document. The Gemara taught that Shmuel agrees that if a woman brings a loan document into the marriage (Kesuvos 85b), or if a Shechiv me'Ra (one who fears lest he die from his illness) gave a loan for a gift (Bava Basra 147), he or she cannot pardon it. It did not say that he admits about Ma'amad Sheloshtan! The Gemara always says that Ma'amad Sheloshtan acquires. It never mentions that the giver can pardon! This is difficult for the Ramban. His answer in Kidushin (the Gemara did not need to say that the latter opinions hold that they argue similarly about a Milveh Al Peh) is poor. I can answer all this, unlike the Rishonim. A loan document is not acquired through Ma'amad Sheloshtan, even the debt itself. It is worse than a Milveh Al Peh. Ma'amad Sheloshtan transfers a Milveh Al Peh, for the borrower is totally freed from the lender. Chachamim enacted as if he paid him, and the lender gave to the recipient, who then lent back to the borrower. When there is a document, as long as the lender or one in his stead holds it, the borrower cannot say 'I paid.' Even if the lender gives the document to the buyer, since Ma'amad Sheloshtan does not transfer the power of the document, the recipient must rely on the lender to use the document. We cannot say that it is as if he paid the lender, for then the document would be Batel. It is unreasonable to say that Ma'amad Sheloshtan totally erases the old loan with a document, and creates a new Milveh Al Peh. Ma'amad Sheloshtan is a Halachah without a reason. We may not add to it! If the document was torn, the loan becomes a Milveh Al Peh. Only then Ma'amad Sheloshtan can work. This is why Kidushin 47b discussed Ma'amad Sheloshtan only regarding a Milveh Al Peh. The questions from Bava Kama 89a and Bava Metzi'a 19b are no longer difficult. Those who say that Ma'amad Sheloshtan works for loans with a document say so only for the debt. The recipient is owed a Milveh Al Peh. No one says that Ma'amad Sheloshtan can transfer a document. The Rambam (Hilchos Ishus 5:17) says that Ploni can be Mekadesh a woman by transferring a loan through Ma'amad Sheloshtan. We must say that Ploni cannot pardon it. He also rules (Hilchos Mechirah 6:12) that one who sells a loan document can pardon it! If he holds that Ma'amad Sheloshtan does not help for a loan document, there is no contradiction.

Question (Tumim 54): The Rishonim understood better than we do. The Shach says that Tosfos agrees that Ma'amad Sheloshtan cannot transfer the document. This is wrong. Tosfos (Bava Metzi'a 20a DH Shema) asks why we are not concerned lest she sold the Kesuvah through Ma'amad Sheloshtan. If the buyer gets only a Milveh Al Peh, in any case the husband would be believed to say that he paid him! And even if he is not believed because Kesuvah is a Tanai Beis Din, the Rishonim connote that a borrower is never believed to say that he paid a document sold through Ma'amad Sheloshtan.

Answer (Nesivos ha'Mishpat, Urim 39): The concern is lest she sold the Kesuvah through Ma'amad Sheloshtan and gave the document through another document, in which case she cannot pardon the debt.

Note: One is not believed to say that he paid a loan document, for he should have asked for the document. Perhaps one is not believed to say that he paid a document sold through Ma'amad Sheloshtan, for he should have asked for the document or a receipt, lest the lender give the document to the buyer!

Tumim (54): We would expect the buyer's lien to be from the day he bought. However, Ma'amad Sheloshtan is a Halachah without a reason; perhaps they enacted a lien from the date of the initial loan. Why did Tosfos struggle to answer the questions from Bava Kama and Bava Metzi'a? We could say that the Beraisos are like Chachamim, who hold (Kidushin 48a) that Ma'amad Sheloshtan does not apply to loans (even though this is not the Halachah)!

Nesivos ha'Mishpat (ibid.): The Shach says that Ma'amad Sheloshtan cannot transfer a document. This seems correct. Rav Huna (Kesuvos 86a) counsels one who bought a document to pay the borrower to write a loan document obligating himself to the buyer, so the seller will be unable to pardon it. He did not suggest Ma'amad Sheloshtan, for it does not work after the document was sold, for the lender has no power over the loan (the borrower is not indebted to him). We cannot extend Ma'amad Sheloshtan to new situations!

Gra (82): Ameimar explained that when one borrows the money, he obligates himself to anyone who will be in place of the lender. Even though Ameimar was rejected, we learn that Ma'amad Sheloshtan applies even to documents.

Gra (119): The simple reading of the Gemara in Kidushin supports the Shach, who says that Ma'amad Sheloshtan does not help for any document.