Gemara

(Ameimar): R. Yosi says that Asmachta (an exaggerated promise) is binding. If so, also an Arev (guarantor) becomes obligated (he does not benefit, yet he is liable to pay if the borrower will not). R. Yehudah says that Asmachta is not binding, so an Arev does not become obligated.

Objection (Rav Ashi): We rule that Asmachta is not binding, yet an Arev becomes obligated! Arvus is not Asmachta. The Arev gets Hana'ah (pleasure). Since the lender trusts him (and lends the borrower due to the Arev), he decides absolutely to accept responsibility as if he himself borrowed.

174b: The Halachah is, an Arev is obligated whether or not he (the borrower) has (property).

175b (Mishnah): If an Arev accepted Arvus in a document underneath the signatures, the lender can collect from Bnei Chorin (of the Arev). A case occurred, and R. Yishmael ruled like this.

Ben Nanas: He does not collect at all! If Levi saw Shimon (the lender) choking the borrower, and said 'leave him alone, and I will give to you', Levi is exempt, for Shimon did not lend due to Levi. An Arev is liable if he said 'lend to him, and I will give to you', for the loan was given due to the Arev.

176a: Ravin said that R. Yishmael argues even in a case of choking, and the Halachah follows him even there.

(Rav Yehudah): In a case of choking, if the Arev made a Kinyan to obligate himself, he is obligated.

Inference: If it was not a case of choking, no Kinyan is required! This argues with Rav Nachman.

(Rav Nachman): An Arev of Beis Din does not need a Kinyan.

Inference: A regular Arev needs a Kinyan!

The Halachah is, an Arev at the time of the loan (when the money was given) does not need a Kinyan. An Arev after the loan needs a Kinyan. An Arev of Beis Din does not need a Kinyan. With the Hana'ah that he is trusted, he resolves to obligate himself.

Rishonim

Rambam (Hilchos Malveh 25:1): If Reuven lent to Shimon, and afterwards Levi said 'I am an Arev', or Reuven claimed from Shimon in Din and Levi said 'leave him. I will be an Arev for you', or Shimon was choking Reuven to make him pay, and Levi said 'leave him alone, and I will give to you', Levi is exempt. This is even if he said 'I am an Arev in front of Beis Din.' If they acquired frm him for this money, in all these cases, whether in front of Beis Din or in front of the lender, he is obligated.

Rambam (2): If Levi said at the time of the loan 'lend to him, and I am an Arev', he is liable. No Kinyan is needed. Similarly, if Beis Din made him an Arev, he is liable even if they did not acquire from him. E.g. if Beis Din wanted to collect from a borrower, and Levi said 'leave him. I will be an Arev for you', since he enjoys Beis Din's trust in him, through that Hana'ah he obligates himself.

Teshuvas Rosh (69:7, cited in Beis Yosef Sof Siman 126): Reuven bought grain from Shimon for a set price, and Shimon wrote a loan document against him. Before acquiring (liability), Reuven asked who will give the grain, and Shimon said 'Yehudah will.' Yehudah agreed, and Reuven accepted (the debt). Now, Reuven demands the grain, and Yehudah says 'when I accepted, I did not have anything of Shimon's. Also, if I must give to Reuven, I need not give more than the value of the grain when Reuven bought it, not at the higher current price.' The law is, since Reuven did not want to obligate himself until Yehudah accepted to give the grain, this is like 'give to him, and I will pay you', which is a Kablan, i.e. an Arev whom one may demand payment from him before approaching the borrower.

Hagahos Ashri (Bava Kama 9:16): R. Yo'el says that a total professional (who erred about the validity of a coin) is exempt even if he said 'see, I rely on you.' An amateur is liable only if he said this. R. Efrayim says that if Ploni asked about a coin, and a professional said that it is valid, Ploni must accept it, therefore he is liable even if Ploni did not say 'see, I rely on you.' One need not buy or accept a security (so one who gave mistaken judgment about these matters is exempt, unless he was told 'see, I rely on you').

Shiltei ha'Giborim (Bava Basra 80b 2): If Reuven wanted to lend to Shimon and asked Levi if he is well-off and proper to lend to him, and Levi said that he is good, and it turned out that Shimon had no property and the loan was lost, if Reuven said to Shimon (it seems that this should say 'to Levi' - PF) 'see, I rely on you', he is liable for the full loss. If he did not say this, R. Yo'el would obligate, and R. Efrayim would exempt.

Poskim

Shulchan Aruch (CM 129:2): If Levi said at the time of the loan 'lend to him, and I am an Arev', he is liable. No Kinyan is needed.

Beis Yosef (DH Kasuv): A Teshuvah in Sefer Chazeh ha'Tenufah says that if a seller wanted an Arev, and Moshe agreed to be an Arev after Ploni already put on the garment, this is not considered an Arev after the money was given. Since he did not want to sell without an Arev, Ploni did not acquire it through wearing it until Moshe agreed to be an Arev.

Beis Yosef (DH Kosav b'Mordechai): The Mordechai (Bava Basra 652) says that if the lender relied on the Arev and returned the document or security, this is like an Arev at the time of the loan.

Prishah (1 DH u'Mah she'Chosav Af): The Gemara explained why Arvus works at the time of the loan, even though it is Asmachta. Why does it help later through a Kinyan? Asmachta acquires only in an esteemed Beis Din! It seems that even after the loan, he gets some Hana'ah from the fact that he is trusted, and this joins with the Kinyan, to be like much Hana'ah at the time of the loan, which works without a Kinyan. We can prove this from an Arev for Beis Din, which works after the loan without a Kinyan, due to the Hana'ah that Beis Din trusted him.

Rema: This is even if he did not explicitly become an Arev, just he told David 'lend to him, for he is Batu'ach (financially secure)', and David relied on him and did so, and it turned out to be false. Levi is liable, for it is as if he was an Arev.'

SMA (7): R. Yerucham explains that this is when he was not Batu'ach at the time Levi said this. If he was Batu'ach, and afterwards faltered, Levi is exempt.

Shach (7): The Yam Shel Shlomo (Bava Kama 9:24) says that Levi is exempt unless Reuven said 'see, I rely on you.' This is even according to the opinion that obligates a moneychanger who erred (and said 'accept this coin. It is valid') even if he did not say 'see, I rely on you', for here Levi did not need to accept liability. R. Yerucham says that he is not less liable than a moneychanger. The Rema (306:6) rules that a moneychanger must say this (to be liable). Why didn't he say so here? This is especially difficult according to the Yam Shel Shlomo, who says that here all agree; the Mordechai and Hagahos Ashri (Bava Kama 116 and 9:16) hold like the Yam Shel Shlomo. We must say that the Rema's words 'he relied on him' mean that there was evidence that he relied on him, and he knew this. The Rif (Bava Kama 35b), Rambam (Hilchos Sechirus 10:5) and Shulchan Aruch (306:6) say (about a moneychanger) that such evidence is like saying 'see, I rely on you.' The Bach says that here, he actually said so. The Rema and R. Yerucham connote otherwise. In any case if he (the lender) knows (the borrower) well, e.g. he is his brother or partner who knows his affairs and principal and that he does not owe others, even if he said 'see, I rely on you', he is exempt.

Shach (8): Levi should not have said that Shimon is Batu'ach, for one can never know this, even if he is his son or father, even if he saw that he has much gold and silver and coins. Perhaps it is not his, or he owes others. However, if brothers shared a big inheritance and he knows that his brother did not owe anyone beforehand, or he was his partner for a while and knew his partner's share and that he did not owe anyone, and later it became known why he lost his wealth, and he [did not] know, this is like Ones, and he is exempt even if he said 'see, I rely on you'. He need not swear Heses that he did not know, for the lender cannot claim 'you surely knew'. All the more so he is exempt if he was Batu'ach and later faltered.

Shulchan Aruch (ibid): Similarly, if Beis Din made him an Arev, he is liable even if they did not acquire from him. E.g. if Beis Din wanted to collect from a borrower, and Levi said 'leave him. I will be an Arev for you', since he benefits from Beis Din's trust in him, through that Hana'ah he obligates himself.

SMA (8): This is even not at the time of the loan, and without a Kinyan.