Your questions are very important ones. In the following response, I address the four major points that you raise.

I. What is the act of Kinyan for a Milveh Al Peh?

There is in fact no Kinyan for a Milveh Al Peh because, by definition, it is a transaction that is agreed upon merely orally. However, if there is no Ma'aseh Kinyan, then what obligates the borrower to pay back his debt?

1. Tosfos in Kesuvos (56a, DH Harei, 3 lines from the bottom of the page) writes that even the opinion that "a debt written in the Torah is considered equivalent to a debt written in a Shtar" agrees that this applies only to debts like damages or Pidyon ha'Ben which, without the Torah's obligation, we would not have been able, with logic, to require the person to pay. In the case of damages, the Torah requires that the perpetrator pay from the best of his property, something that logically we would not have required him to do. In the case of Pidyon ha'Ben, the Torah requires that a man pay five Shekels to redeem his son, which, again, is not a requirement that we would have known from logic. Tosfos writes that in contrast, the obligation to return a loan does not originate from the Torah because it is logical that one must pay back what he borrowed. Every normal legal system requires that one return a loan.

2. The Rashba in Kidushin (13b, DH Milveh Kesuvah b'Torah, second paragraph) elaborates on this and writes that even though the obligation to return a loan is actually written in the Torah (Devarim 24:11: "And the man to whom you have lent will bring to you the collateral outside"), the obligation to pay does not take effect as a result of the Torah's words, but rather as a result of the borrower's own actions when he took the loan. By his act of borrowing, the borrower himself automatically committed himself to pay back.

In short Tosfos' opinion is that logic demands that one must pay back the loan even though the transaction and commitment was only verbal.

B. There is another way of understanding the obligation to return a loan even though there was no Kinyan. The Rambam (Hilchos Malveh v'Loveh 26:10) writes that a minor who borrowed money is obligated to return the loan when he reaches adulthood. The Magid Mishneh there explains that although the Halachah is that if a minor damaged property or injured someone, he is exempt from paying when he becomes an adult (Bava Kama 87a, Shulchan Aruch CM 424:8), nevertheless when he borrowed money he acquired benefit from the loan, and thus it follows that he must pay for this benefit.

This may be compared to the case of the Gemara in Kesuvos (80a) in which Reuven enters Shimon's field and plants it without permission. The law is that Shimon must pay for the benefit he received. This teaches that if one gained as a result of someone else's actions, one must pay for this benefit. Similarly, one who borrows money benefits, and thus it follows that he must return the money, since one always must pay for benefit received.

C. There is a third way of understanding why one must return the Milveh Al Peh even though there was no Kinyan. The Rashba in Teshuvos (3:56) cites the Gemara in Kesuvos (86a) which states that "it is a Mitzvah to pay back a loan." (What exactly is the Mitzvah, that obligates one to pay back a loan, is subject to discussion.)

D. In general, one cannot effect a Kinyan merely by Dibur, speech. This is stated many times in the Gemara. One example is Bava Metzia 94a, where the Gemara says that a Shomer Sachar can make a condition upon himself and agree to raise his level of responsibility to that of a Sho'el. The Gemara asks how he becomes obligated if his commitment was only "Devarim b'Alma," speech in general. Mere words cannot make a Kinyan to change one's level of guardianship.

E. In conclusion, even though normally one does not commit oneself by mere words, the case of Milveh Al Peh is different, either because it is simply logical that if one borrowed one must pay back, or because of the financial benefit one gained for which one always must pay, or because of the Mitzvah of the Torah to pay back.

II. What is the act of Kinyan for Mechilas Milveh, forgiving a loan?

A. The Rambam (beginning of Hilchos Mechirah 1:1) writes, "A sale is not transacted by mere words," and he proceeds to detail how each Kinyan works. However, later in Hilchos Mechirah (5:11), the Rambam writes, "There are many things which do not require a Kinyan, and there is no reason to use a Kinyan for them.... For example, if someone forgive a debt or collateral that he possesses in his hand."

B. The Rambam returns to this topic in Hilchos Zechiyah u'Matanah (3:2) and writes, "If he was Mochel to his friend a debt which he had upon him, or he gave him a collateral which had been deposited with him, this is a gift which is acquired by words alone and nothing else is required, as we have explained."

We see that the Rambam emphasizes that foregoing a loan is an exception to the general rule that a transaction always needs a Kinyan.

C. The question is, why does Mechilas Milveh not require a Kinyan? The answer to this question is based on the definition of Mechilah as stated by the Rashba in Teshuvos (#1033), cited by the Beis Yosef (CM 195, paragraph #14 in the new printings). The Rashba defines Mechilah as merely a "Siluk" (literally, "removal"). This means that when the lender is Mochel the debt, he in effect is saying to the borrower that he removes himself from the debt and no longer has any connection with it. This explains why no Kinyan is necessary. Once the lender removes himself from the debt, the money owed to him is automatically released.

D. Another way of understanding Mechilah is found in the Rishonim. The Ritva in Kidushin (16a, DH Amar Rabah) writes that Mechilah is comparable to a Matanah, a gift. According to the Ritva, why does Mechilah not require a Kinyan in the same way that a gift requires a Kinyan?

This question may be answered based on the Gemara in Kidushin (47a) which states that "Milveh l'Hotza'ah Nitnah": the money of a loan is given to be spent. That is, when Reuven lends money to Shimon, he does so in order for Shimon to spend the money. Obviously, Shimon is not required to return the same bills or coins that he borrowed. This means that the money is now in Shimon's possession, and Shimon merely has an obligation to return the loan. Therefore, when Reuven is Mochel the debt, he is transferring the money to Shimon's ownership through a Kinyan Chatzer; since the money is presently in Shimon's domain, he acquires the money when Reuven merely says that the money can remain there. According to the Ritva, the Kinyan on Mechilas Milveh is a Kinyan Chatzer (whatever is in a person's house or property belongs to him); no special Kinyan is required in this case because Reuven merely says to Shimon that the money in his domain can remain there.

III. What is "Ma'amad Sheloshtan" needed in order to transfer a loan to someone else?

Rashi in Kidushin (48a DH b'Ma'amad) writes that the concept of Ma'amad Sheloshtan does not one in the same way as ordinary monetary transactions. Rashi notes that the Gemara in Gitin (14a) calls Ma'amad Sheloshtan one of three "Halachos without a reason." What happens in a transaction of Ma'amad Sheloshtan is that Shimon owes money to Reuven, and Reuven decides that he wants Shimon to pay back the money not to him, but to Levi. In order to transfer the right to collect the loan to Levi, Reuven must say to Shimon in the presence of all three parties that he should pay the money back to Levi.

This does not conform with the general principle (which we cited in the previous answer, in the name of the Rambam in Hilchos Mechirah 1:1) financial transactions are not effected by words alone but generally require some act of Kinyan. According to the standard rules, Reuven cannot commit himself to allow Levi to collect the loan merely through speech, which is why a special enactment of the Rabanan was required in order to enable the transaction of Ma'amad Sheloshtan to work.

In contrast, when Reuven lends money to Shimon through a Milveh Al Peh, it is clear that the loan takes effect by virtue of the transfer of the actual money. The discussion above in the first answer was what obligates the borrower to repay. What makes the loan itself, however, is clear. On the other hand, to obligate Shimon to pay back to Levi instead of to Reuven requires a special institution of Ma'amad Sheloshtan. This answers your question of why Ma'amad Sheloshtan is required and it does not suffice merely to do the same transfer as Milveh Al Peh.

(See also Rashash to Bechoros 18b for more examples of commitments that can be created even without a Kinyan.)

IV.

A. Finally, you asked whether the obligation to return a loan is mid'Oraisa or mid'Rabanan. The simple answer is that it is a Torah obligation, because it is logical that one must return what he borrowed, as we explained above in the first answer. However, I found an explanation in the writings of Rav Shimon Shkop which sheds more light on this question.

Rav Shimon Shkop (Sha'arei Yosher, Sha'ar 5, ch. 2, DH ul'Aniyus Da'ati) writes that there is a concept, which he calls a "Chiyuv Mishpati" (literally, a legal obligation), which we would know even without the Mitzvos of the Torah. Once logic determines that Shimon must return the money that he borrowed from Reuven, the Torah adds that there is a Mitzvah to return the loan.

Rav Shimon Shkop compares this to our obligation and need to serve Hash-m. It follows logically (since Hash-m created us) that we must do what He asks of us. Once we acknowledge Hash-m, it is clear that we must keep the Mitzvos of the Torah.

V. In summary, there are situations in which mere speech is equivalent to an act of Kinyan. In those cases, a Kinyan is unnecessary ("ha'Dibur b'Zeh k'Kinyan," in these cases speech is like a Kinyan).

Therefore, when the lender is Mochel the loan, since a Kinyan is not necessary (for the reasons explained above), it follows that when he merely says to the borrower that he is foregoing the debt, it is as though he made a Kinyan. Even though for most laws of the Torah an actual Kinyan is required, there nevertheless are certain cases in which a Kinyan is unnecessary. In those cases, speaking is just as effective as doing a positive act.

This idea is expressed in Gitin (67a). The Gemara states that witnesses are suspected of telling other witnesses to sign a false deed, but they are not suspected of signing falsely themselves. The Gemara says that people may speak wrongly but they are not suspected of actively committing a fraud. Tosfos there (DH Dibura) writes that sometimes words themselves inevitably cause a Ma'aseh to happen. In such a case, we say that the Dibur itself is equivalent to a Ma'aseh. (See also Teshuvos Sho'el u'Meshiv 1:3:67 DH v'Hineh.)

Kol Tuv,

Dovid Bloom