1) PROOF TO ONE'S OWNERSHIP OF A "SHTAR"
QUESTION: The Gemara gives a number of ways to reconcile the view of the Tana of the Mishnah with the view of the Tana of the Beraisa. The Tana of the Mishnah says that either one of the two men named Yosef ben Shimon may present a Shtar Chov against another party (and we are not concerned that it fell from the other Yosef ben Shimon and this one found it). The Tana of the Beraisa argues and says that neither one of the two men named Yosef ben Shimon may present a Shtar Chov against another party, because the debtor may claim that the Shtar was written for the other man with the same name.
The Gemara first suggests that the Tana'im disagree about whether "Osiyos Niknos b'Mesirah." The Gemara then suggests that both Tana'im agree that "Osiyos Niknos b'Mesirah," and they disagree about whether or not the bearer of the Shtar needs to bring proof that it is rightfully his. The Tana of the Mishnah maintains, like Rava, that he does not need to bring proof that the Shtar is his. The Tana of the Beraisa maintains, like Abaye, that he does need to bring proof that the Shtar is rightfully his (either that the Shtar was written for him in the first place, or that it was given to him by the original lender in order to transfer ownership of the debt to him).
Why should the bearer of the Shtar need to bring proof that the debt belongs to him, according to Abaye and the Tana of the Beraisa? The Gemara at this point maintains that there is no concern that the Shtar fell from one Yosef ben Shimon and the other Yosef ben Shimon found it. Hence, since this Yosef ben Shimon is in possession ("Muchzak") of the Shtar, there is no reason to doubt that he is the owner of the Shtar, just as there is no reason to doubt that any other item of Metaltelin in his possession belongs to him. (RASHBA)
ANSWERS:
(a) The Gemara later (end of 173a) teaches the rule that "Yad Ba'al ha'Shtar Al ha'Tachtonah" -- the bearer of the Shtar has the "lower hand" and must bring proof in a case of a doubt (because of the principle, "ha'Motzi me'Chaveiro Alav ha'Re'ayah"). The Tana of the Mishnah and the Tana of the Beraisa disagree about whether there is a significant doubt in this case such that the rule of "Yad Ba'al ha'Shtar Al ha'Tachtonah" applies. The Tana of the Beraisa maintains that the possibility that the other Yosef ben Shimon deposited this Shtar with the Yosef ben Shimon who is now holding it, or the possibility that this Yosef ben Shimon stole the Shtar from the other Yosef ben Shimon, is enough of a concern (despite the low probability that it occurred) to create a doubt about the true ownership of the debt, even though one Yosef ben Shimon is presently holding the Shtar. The Tana of the Mishnah, on the other hand, maintains that this is not a substantial enough concern to create a doubt, since it is highly unlikely that the other Yosef ben Shimon deposited his Shtar with this Yosef ben Shimon, or that this Yosef ben Shimon stole it from the other one. (See RI cited by TOSFOS in Yevamos 116a, DH Osiyos.)
(b) The RASHBA suggests that the Tana of the Beraisa maintains that being in possession ("Muchzak") of a Shtar is not the same as being in possession of other mobile property (where the very fact that he is holding the item is proof that it belongs to him). All other items are made to be used themselves, and, therefore, when a person hands over an item to another person, that act signifies a total transfer of ownership. The item has no other usage or purpose other than the use obtained from the body of the item itself. A Shtar is different. The use of a Shtar does not come from the actual paper of the document. Rather, the purpose of a Shtar is to represent a Shibud, an obligation, which one person has to another person. Therefore, the fact that one person is holding the Shtar does not show that he is in possession of the Shibud that the Shtar represents, even though his possession of the Shtar shows that he owns the piece of paper (and is entitled to use it as a bottle stopper if he wants). Only when he brings proof that the Shtar was written for him, or that it was given to him with the words, "Acquire hereby this Shtar and the Shibud in it," is there certainty that the Shibud itself belongs to him as well.
In contrast, Rava and the Tana of the Mishnah maintain that the bearer's possession of the Shtar suffices as proof that the Shibud was given to him. Since he is holding the Shtar, we assume that the Shtar was given to him in order to give him ownership of the debt written therein, as people usually do not give others a Shtar merely for the purpose of stuffing their bottles with it.
(c) The RASHBA gives another answer (and refutes his first one) and says that the case of a Shtar in the hands of a person who has the same name as another person in that town is different. We may assume that the bearer of the Shtar made sure that witnesses were present when the Shtar was written for him, or when it was given to him by the other Yosef ben Shimon, in anticipation of the debtor's future challenge to his rightful ownership of the debt. Therefore, he must present those witnesses when he wants to collect with this Shtar.

173b----------------------------------------173b

2) THE SOURCE FOR THE "SHIBUD" OF AN "AREV"
QUESTION: Rav Huna asks what the source is for the law that an Arev is able to be Meshabed himself through his word alone, without a formal act of Kinyan, so that he becomes obligated to pay back the debt of the borrower. Rav Huna answers that the source for the Shibud of an Arev is the verse, "Anochi E'ervenu, mi'Yadi Tevakshenu" -- "I will guarantee him, from my hand you can demand him" (Bereishis 43:9).
Rav Chisda challenges Rav Huna's source by pointing out that the verse he quotes is discussing a case of an Arev Kablan and not an ordinary Arev, and thus he cannot learn from that verse that an ordinary Arev is able to be Meshabed himself through his word alone. Rav Chisda proves that the verse that Rav Huna cites is referring to an Arev Kablan by citing another verse, "Tenah Oso Al Yadi va'Ani Ashivenu Elecha" -- "Give him into the care of my hand, and I will return him to you" (Bereishis 42:37). Since he uses the words, "Give... and I will return...," he clearly is making himself an Arev Kablan, because this is the phrase used by an Arev Kablan to commit himself, as the Gemara later teaches (174a).
How does Rav Chisda disprove Rav Huna's source from the verse of "Anochi E'ervenu" (Bereishis 43:9) from the earlier verse, "Tenah Oso Al Yadi" (Bereishis 42:37)? In the verse that Rav Huna quotes, it is Yehudah who is committing himself to be an Arev for the return of Binyamin to his father, Yakov. In the verse that Rav Chisda quotes, it is Reuven who is committing himself to be an Arev for the return of Binyamin! Even if Rav Chisda is correct that Reuven made himself an Arev Kablan, how can Rav Chisda prove from there that Yehudah was an Arev Kablan?
ANSWER: The MAHARSHA explains that it was Reuven who first implored Yakov, unsuccessfully, to send Binyamin with him to Mitzrayim. If Reuven -- who offered to take responsibility for Binyamin with the degree of an Arev Kablan -- was unsuccessful, then it does not make sense that Yehudah, who urged Yakov to send Binyamin with him after Reuven's unsuccessful attempt, would offer to be only an ordinary Arev. It must be that Yehudah, too, was an Arev Kablan. (See also TORAH TEMIMAH to Bereishis 43:9.)
What, then, was Rav Huna's Havah Amina when he quoted the verse discussing Yehudah's offer to be an Arev, if it is obvious that he was offering to be an Arev Kablan?
The IYUN YAKOV suggests that Rav Huna understands that there was a difference between the two offers of Arvus from the fact that they are phrased with two different expressions (as the Gemara on 174a teaches, each type of Arvus is accepted through a different expression). In addition, the Iyun Yakov suggests, Reuven did not really intend to be Meshabed himself, as he did not actually use the word "Arev" or "Kablan." Yehudah, though, had absolute intent to accept full responsibility for Binyamin, as he himself said later to Yosef (Bereishis 44:32).
Alternatively, perhaps Rav Huna understands the verses in the manner that RASHI explains them, or in the manner that the RAMBAN explains them. RASHI (Bereishis 42:38) explains that Yakov rejected Reuven's offer because Reuven had offered to give up the life of two of his children if he did not return Binyamin to his father. Yakov responded, "Are they your children and not my children?!" Yehudah, on the other hand, offered his own eternal life in Olam ha'Ba if he did not return Binyamin to his father. Therefore, it sufficed for him to commit to be an Arev, and not a Kablan.
The RAMBAN explains that the reason why Yakov did not accept Reuven's offer, but did accept Yehudah's offer, was because he trusted Yehudah with Binyamin much more than he trusted Reuven. Yehudah was the leader of and the strongest of the brothers; they certainly would listen to him and heed his word to take care of Binyamin. Reuven, however, had already lost some of Yakov's trust with the earlier mishap of the bed of Bilhah (Bereishis 35:22). In addition, Yehudah strategically waited to ask Yakov to send Binyamin to Mitzrayim until all of the food in Yakov's home had been completely consumed (Bereishis 43:2). Therefore, it could be that Yehudah only needed to offer to be an Arev, and not even an Arev Kablan, in order to persuade Yakov to send Binyamin with him.
3) BECOMING AN "AREV" FOR WHOM
QUESTION: Rav Huna asks what the source is for the law that an Arev is able to be Meshabed himself through his word alone, without a formal act of Kinyan, so that he becomes obligated to pay back the debt of the borrower. Rav Huna answers that the source for the Shibud of an Arev is the verse, "Anochi E'ervenu, mi'Yadi Tevakshenu" -- "I will guarantee him, from my hand you can demand him" (Bereishis 43:9). The Torah there relates Yehudah's commitment to be an Arev for the return of Binyamin to his father, Yakov.
Who, though, was the "Loveh," the one who "borrowed" Binyamin, in that case, such that Yehudah obligated himself to be an Arev for that person? It seems that Yehudah himself was the "borrower" who was taking Binyamin from Yakov! How could Yehudah become an Arev for himself, for his own debt?
ANSWERS:
(a) The RITVA answers in the name of his teachers that since this verse is only an Asmachta for the law of an Arev, it does not need to be identical to the case that the Gemara attempts to learn from it.
(b) The RITVA himself maintains that Rav Huna is not quoting the verse merely as an Asmachta, but as an actual source for the law that an Arev can obligate himself with his word alone. The Ritva explains, therefore, that Yehudah was making himself an Arev for Reuven and the other brothers. He was asking Yakov to hand over Binyamin to the other brothers (who would be considered the "borrower"), and he would be their guarantor to ensure that they return Binyamin to Yakov.
It seems that the Ritva means to say that Yehudah was making himself an Arev for the brothers who were acting as the Shali'ach of the borrower (and not that they themselves were the "borrower"). It was Yosef, in Mitzrayim, who was the one "borrowing" Binyamin, and who had sent the brothers as his agent to bring Binyamin to him. Thus, Yehudah was actually making himself an Arev to Yosef for the return of Binyamin to his father.