We have learned above (see dapim 173 and 174) about the role of an arev – someone who agrees to guarantee a loan. According to the Mishna (175b) if the arev signed on the loan after it had already been agreed upon, signed and witnessed then even in cases where he will have to pay, there would be no liens on his property, and collection could only be made from money that was in his possession at the time of collection.
The Mishna relates that such a case came before Rabbi Yishmael, and that was the ruling that he offered. In response, Ben Nannas argued that an arev who signs the note after the witnesses will not be obligated to pay at all. He compared it to a case where someone came across a scene in the marketplace where a lender was choking someone who had borrowed from him and was demanding his money back. If that person stepped in and offered to pay the debt so that the borrower would be saved, surely he is not obligated to do so. Ben Nannas explains that in such a case the loan was originally made with no expectation that there was an arev involved, and no obligation was created at a later time with a simple verbal commitment. According to the Mishna, Rabbi Yishmael was so taken by this argument that he recommended that anyone who wanted to develop his intelligence should study the monetary laws, and whoever wanted to truly understand the monetary laws should become a student of Ben Nannas.
On our daf, the Gemara brings Rabba bar bar Ḥana who quotes Rabbi Yoḥanan as saying that Rabbi Yishmael may have offered praise to Ben Nannas, but that that halakha follows Rabbi Yishmael’s position. In fact, Rabbi Yaakov quotes Rabbi Yoḥanan as saying that Rabbi Yishmael disagreed with Ben Nannas even in the case of the choking lender in the marketplace. Thus, Rabbi Yishmael’s position was that an arev can fully commit himself to guarantee someone’s loan, even if it is not at the time that the loan was made. Such a guarantee, however, cannot create a lien on his property.