OPINIONS: The Gemara (end of 173b) says that according to the Tana Kama of the Mishnah, a lender may approach an Arev Kablan to pay even if the borrower has property and the lender has not yet asked the borrower to pay. Raban Shimon ben Gamliel maintains that the lender must approach the borrower first; he may collect from the Arev (even an Arev Kablan) only when the borrower has no property with which to pay.
The Gemara continues its discussion about an Arev Kablan and lists the different expressions that a guarantor may use to accept upon himself an obligation of Arvus or an obligation of Kablanus.
What determines whether an Arev is an "Arev Kablan"?
(a) RASHI in Gitin (49b, DH b'Kablan; see also RASHBAM earlier in Bava Basra, beginning of 47a) explains that the word "Kablan" denotes that the Arev received ("Kibel") money or an object on behalf of the borrower. Since the object passed through his hands, he has accepted a greater obligation towards the lender. TOSFOS there (Gitin 49b, DH b'Kablan) explains that according to Rashi, this is the meaning of "Kablan" whenever the Gemara discusses the concept.
In the case of a normal loan, it is clear that the Arev can become a Kablan by accepting the money that the lender gives to the borrower. However, the Gemara later (174b) and in Gitin (49b) says that when a person becomes an Arev for the payment of a Kesuvah, he is not obligated to pay even if the husband has no money with which to pay (as per the reason that the Gemara here later gives). If he becomes an Arev Kablan for a woman's Kesuvah, then he does become obligated to pay. How, though, does the Arev Kablan obligate himself in the case of a Kesuvah according to Rashi? In the case of a Kesuvah, the woman has not yet received her Kesuvah, and she gives nothing to the Arev Kablan that would obligate him to pay the Kesuvah when the time comes. How, then, can a person become a Kablan for a Kesuvah?
Rashi explains that one can become a Kablan for a Kesuvah in a case where the husband designates certain Metaltelin items in his possession for the payment of the Kesuvah, and he gives them to his wife to hold until the time comes to collect the Kesuvah. If the wife then hands over those Metaltelin to an Arev in order for him to return them to her husband, then the Arev becomes an Arev Kablan.
In contrast to Rashi's definition of an Arev Kablan, the Gemara here gives different criteria for defining the difference between a Kablan and an Arev. The Gemara says that when a person asks the lender to accept him as a Kablan for the loan, he uses a different wording ("go give him money") than the wording used by an Arev ("lend him money"). Tosfos explains that Rashi understands that a Kablan requires both a different wording and physical acceptance of the money of the loan for the borrower. (See also Insights to Gitin 49:2.)
(b) The RASHBA and RITVA in Gitin explain that a Kablan becomes a Kablan simply because of the wording that he uses when he accepts the Arvus. This follows the simple reading of the Gemara here in Bava Basra. It is not necessary for him to physically accept any money or object. The reason he is called a Kablan is that he has accepted ("Kibel") responsibility to pay back the loan. If he physically accepts the loan itself, then he has a much greater obligation than an Arev Kablan, and the lender may collect only from him and not from the borrower (as the Gemara here says).
The RASHBAM (173b, DH Kablanus) seems to have retracted what he wrote earlier in Bava Basra and sides with these Rishonim.
(c) The TOSFOS HA'ROSH and the RASHBA (in the name of Tosfos) in Gitin compromise and explain that a person normally becomes a Kablan based on the wording that he uses, and not because he physically accepts the loan. However, the wording alone is not enough to make a person a Kablan for a Kesuvah (because he does not cause the wife any loss by accepting responsibility, since she has not handed over any money as a result of his promise to pay back), which is the case discussed in Gitin. Therefore, in order to become an Arev Kablan for a Kesuvah, it is necessary for the wife to receive Metaltelin that were designated for the payment of her Kesuvah and to return them based on the promise of the Arev. Only then does one become an Arev Kablan.