Gemara

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

(Rava): Presumably, Rav's law applies only to a deposit (because he could give it over), but not to a loan. However, Rav actually said it even about a loan!

(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.

Question: Why does this work?

Answer #1 (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 one born after the loan was given! Even R. Meir, who says that one can transfer ownership of something not yet in the world, requires the receiver to be in the world!

Answer #2 (Rav Ashi): Rather, through the benefit of converting an old loan into a new loan, Shimon is able to obligate himself to Levi.

Question (Huna, son of Rav Nechemyah): If so, Rav's law would not apply if Levi were like people of Bar Elyashiv's house, who force one to pay immediately! (It is not beneficial to transfer a loan to such people. It is unreasonable to say that the law varies according to the recipient!

Answer #3 (Mar Zutra): Rav's law is an enactment without reason (how it works).

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 8b (Beraisa): If Levi was Mekadesh with a security, she is Mekudeshes.

This is when Levi had a security for a loan that Ploni owed to him, and gave the loan and the security for 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.

Question: What do they argue about?

Answer #1: Regarding a loan document, they argue like Rebbi and Chachamim:

(Beraisa - Rebbi): One acquires a document when it is handed to him;

Chachamim say, he acquires it only if the giver also writes a second document to transfer ownership of the first document.

Answer #2: R. Meir and his Chachamim all hold like Rebbi. They argue about Rav Papa's law (that one who sells a document must write (or say) 'acquire it, and all the liens it has.')

Answer #3: All hold like Rav Papa. They argue about Shmuel's law (that one who sold his loan document can pardon the debt). Chachamim hold like Shmuel (she does not resolve to accept Kidushin, for she fears lest he pardon the debt).

Answer #4: Both Tana'im hold like Shmuel. R. Meir holds that a woman resolves to accept the document. She does not fear lest he pardon the debt.

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.

Rishonim

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.

Rosh (Gitin 1:17 and Bava Metzi'a 1:48): Normally, if Reuven sold a debt (owed by Shimon) to Levi, and later pardoned it, Levi cannot collect. If he sold through Ma'amad Sheloshtan, he cannot pardon it. We said that Tana'im argue about whether a woman accepts a loan document for Kidushin, or if she fears lest he pardon the debt. Regarding a Milveh Al Peh, they argue about whether or not Ma'amad Sheloshtan applies to loans. If Reuven could pardon, even if it applied to loans, Chachamim would say that she is not Mekudeshes! We did not conclude that all agree that it applies to loans, and they argue about whether or not she fears lest he pardon. In Ma'amad Sheloshtan, Reuven's Shibud (lien) on Shimon is totally uprooted, (not only his lien on the property, rather,) even Shibud ha'Guf (Shimon's obligation to pay). There is a new Shibud to pay Levi. If one sells a loan document not in front of the lender, no Kinyan can transfer Shibud ha'Guf. Ma'amad Sheloshtan was enacted to help business. It does what no Kinyan can do. If Reuven could pardon, the enactment would not help.

Rebuttal (Ramban Bava Basra 144a DH va'Ani): There is no proof from Kidushin. The Gemara explained the argument about a Milveh Al Peh according to a different answer regarding a loan document. Indeed, the one who says that they argue about Shmuel's law regarding a loan document, says the same about a Milveh Al Peh! The Beraisa teaches the argument about Shmuel's law in two cases. Chachamim hold that the seller can pardon even when there was a document. R. Meir holds that he cannot pardon even if it was a Milveh Al Peh. Shmuel taught Stam, i.e. in every case, that the seller can pardon. If there were a distinction, the Ge'onim would have said so.

Ramban (Kidushin 48a DH b'Milveh): The Rif did not bring that if one was Mekadesh with a loan (owed by someone else) through Ma'amad Sheloshtan, she is Mekudeshes. This is because he can pardon it. We find (Kidushin 8b) that one can be Mekadesh with another's loan by giving the security. We did not say that it suffices to give through Ma'amad Sheloshtan!

Ran (Teshuvah 64): The Ramban strove to show that one can pardon a debt given through Ma'amad Sheloshtan to explain why the Rif omitted this. The proof from Kidushin 8b is not a proof. It taught that a security helps even without Ma'amad Sheloshtan. Also, Ma'amad Sheloshtan helps only if the borrower agrees, but a security helps even against his will. Therefore, I favor the Ri's opinion (that he cannot pardon it), especially in our documents, in which we write a lien to the lender or anyone who will come in his stead.

Poskim

Shulchan Aruch (EH 28:13): If Ploni owed Levi, and Levi told Leah 'you are Mekudeshes to be with the money Ploni owes me', b'Ma'amad Sheloshtan, she is Mekudeshes. Some say that she is not Mekudeshes. Without Ma'amad Sheloshtan, even if there was a document, she is Mekudeshes only mi'Safek.

Beis Yosef (CM 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. Really, most say that in this case he cannot pardon it, including Tosfos, the Rosh and Ran.

Chelkas Mechokek (22): In Choshen Mishpat (66:29), the Shulchan Aruch ruled that one who gave through Ma'amad Sheloshtan cannot pardon. Here, due to the stringency of Kidushin, he is concerned for the opinion that he can pardon, and she does not resolve to be Mekudeshes.

Beis Shmuel (36): Here the Shulchan Aruch is concerned for the other opinion because it is an Isur.