Gemara

(Mishnah): David does not transgress withouseholdlding wages if he arranged with a grocer to pay David's worker (and David will reimburse the grocer).

112a - Question: If the grocer refuses to pay, can the worker claim from David?

(Rav Sheshes): He cannot claim from David;

(Rabah): He can claim from David.

Rabah infers from the Mishnah that David does not transgress if he arranged with a grocer or, but he is obligated to pay. Rav Sheshes explains that it is irrelevant to discuss transgressing, for he is exempt from paying.

Gitin 13b (Rav): If Reuven says to Shimon 'you have money of mine. Give it to Levi', and all three are present, Levi acquires the money.

(Shmuel): If Reuven says to Shimon 'you borrowed money from me. Give it to Levi', and all three are present, Shimon owes the money to Levi.

(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, it should not be possible to give the loan to someone who was not born at the time the loan was given! Rather, through the benefit of converting an old loan into a new loan, Shimon is able to obligate himself to Levi.

Objection (Huna, son of Rav Nechemyah): If so, Rav's law would not apply if Levi's temperament is to force borrowers to pay immediately!

(Mar Zutra): Rather, Rav's law is an enactment without reason (how it works).

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 a lender who sold his loan document can pardon the debt. 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.

Rishonim

Rif: The Halachah follows Rabah, who says that if David arranged with a grocer to pay David's worker (and David will reimburse him), and the grocer refuses to pay, the worker can claim from David. However, Ma'amad Sheloshtan acquires, implying that they cannot retract! Here, the grocer had nothing of David. David wanted him to give on credit. This is why if the grocer will not give, the worker can claim from David. The Yerushalmi says that if David arranged with his workers in front of the grocer, they have no claim against David. This is like Rav Sheshes; the Halachah does not follow him.

Rambam (Hilchos Ishus 5:17): If David owed money to Reuven, and Reuven told Leah that he is Mekadesh her with it b'Ma'amad Sheloshtan, she is Mekudeshes.

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.

Kesef Mishneh: Meforshim argue about whether or not one who sold through Ma'amad Sheloshtan can pardon the loan, and also about whether or not Hamcha'ah (telling one's creditor that Ploni will pay him, in the presence of all of them) exempts the borrower even without specifying. I say that they depend on each other. If Hamcha'ah does not exempt without specifying, it is because the lender still has a grasp on the debt. Therefore, he can pardon it. From Hilchos Ishus (5:17), it is clear that the Rambam holds that the lender cannot pardon. Also his words here 'none of them can retract' suggest this. The Ran says that the Rif omitted the law of Kidushin because he holds that the giver can retract, and she is not Mekudeshes. I say that he did not need to teach it, for from his ruling in Bava Metzia, we know that the giver cannot pardon. Even though the Rosh and Tur say that Hamcha'ah does not exempt (without specifying), but a giver cannot pardon, what I wrote is reasonable.

Rebuttal (Shach CM 126:10): Tosfos, the Ran and Nimukei Yosef citing R. Tam, and the Rema also say that Hamcha'ah does not exempt, but a giver cannot pardon. The Ramban holds oppositely; Hamcha'ah exempts, but a giver can pardon! Perhaps the Kesef Mishneh retracted, for he rules (CM 126:1) that the giver cannot pardon it, but brings (126:9) both opinions about whether or not Hamcha'ah exempts.

Rosh (Gitin 1:17) and Tosfos (Gitin 13b DH Tenehu): One who gave a debt through Ma'amad Sheloshtan cannot pardon it. We did not conclude (Kidushin 48a) that all agree that Ma'amad Sheloshtan applies to loans, and they argue about whether or not she fears lest he pardon the debt. In Ma'amad Sheloshtan, the lender's Shibud is totally uprooted, not only his lien on the borrower's property, rather, even Shibud ha'Guf (the borrower's obligation to pay). There is a new Shibud to pay the recipient. When one sells a loan document not in front of the borrower, no Kinyan can transfer Shibud ha'Guf. Ma'amad Sheloshtan is an enactment to help business. It accomplishes what no Kinyan can do. If the lender could pardon the debt, the enactment would not help.

Prishah (CM 126:10): The Rosh did not simply say that it is as if the buyer appeased the borrower to write a new document obligating himself to him, for he holds that Ma'amad Sheloshtan works b'Al Korcho of the borrower.

Rambam (Hilchos Mechirah 6:12): Kinyan of documents through writing another document is mid'Rabanan. Mid'Oraisa, only the paper can be transferred. Therefore, the seller can pardon the loan. Even his heir can pardon it.

Rebuttal (Ra'avad): No, he can pardon it because the borrower tells the buyer 'I did not obligate myself to you.' If the wrote in the document that he obligates himself to the lender or anyone who will come in his stead, the lender cannot pardon it after selling it.

Rejection (Ramban Gitin 13b DH v'Ika...): If so, Ameimar (who says that a borrower obligates himself to anyone who will be in place of the lender) must hold that a seller cannot pardon. If so, Shmuel contradicts himself! We return a receipt for a Kesuvah (Bava Metzia 19b) without concern lest he agreed to pay anyone in her stead! Rather, in any case, the seller can pardon. Some say that normally, one who sold a debt can pardon it because the sale is only mid'Rabanan, but here he obligated himself (mid'Oraisa) when he borrowed. This is wrong. Chachamim enacted that it is as if he intends for this! Alternatively, this is Ameimar's opinion; the Halachah does not follow him.

Defense (Ran Teshuvah 64): Ameimar did not say that every borrower agrees to pay whoever will be in place of the lender. Ma'amad Sheloshtan works only with the borrower's consent! Surely, Rav Ashi (who says that it works through the borrower's Hana'ah) holds that it is only with his consent. He understood better than we do. Ameimar and Rav Ashi argue only about how Ma'amad Sheloshtan works, not about the Halachos. Ameimar was concise. He meant that one obligates himself to whoever will be in place of the lender, and he will agree to him. The Acharonim agree with the Ra'avad.

Rebuttal (Rosh Teshuvah 69:1): Obligating oneself to whoever will come in the lender's stead explains why the Kinyan takes effect. It does not imply that the lender cannot pardon.

Nimukei Yosef (Bava Basra 67a): The Rif says that once the recipient acquires, he pardoned his claim against the lender. Most Meforshim say that Shmuel's law that one who sold a loan document can pardon it is even if he sold through Ma'amad Sheloshtan.

Poskim

Shulchan Aruch (126:1): If Shimon had Reuven's money, a loan or deposit, and in Levi's presence Reuven told Shimon 'the money you have of mine, give it to Levi', Levi acquires it. Reuven cannot pardon it to Shimon.

Shach (11): The Shulchan Aruch holds that Reuven cannot pardon even if he has property, and Levi will not lose through the pardon (he will collect his debt from Reuven's property), so it is not considered detrimental to others.