Baba Basra 173b - Status of an Areiv (cosigner)

There is a fundamental argument between the ketzos and nesivos (C.M. 129) to explain what binds an areiv to have to pay the debt, if the borrower defaults on the loan. The Ketzos understands from the Rashbam that the lender is considered an agent of the ערב, as if the areiv were lending money to the borrower but asking the lender to lay it out for him. Rashbam writes - ושליחותא דערב קא עביד מלוה, כאילו הוא (הערב) עצמו הלוה. According to this approach, the ketzos explains that an areiv can only be bound to pay the debt if he commands the lender to lend with a language that can be considered the appointment of a shliach. But, if the areiv would say "I will be an areiv for anyone who lends money to Reuven", it is similar to the language of "whoever gives food to Reuven will not lose", which is not considered a language of shlichus and is permitted even if Reuven is mudar hana'ah. Here too, it is not considered a language of shlichus and therefore the areiv would not be bound to pay the debt. However, the Nesivos argues that this cannot be the concept that binds an areiv to pay the debt. We find some tana'im who hold that even if one accepts to be an areiv after the money was lent, he can be misha'abed himself as an areiv. These opinion surely don't consider the obligation of the areiv to be because of shlichus, because the money wasn't lent by the lender as a shliach of the areiv. Even the opinions who say he cannot be an areiv after the money was lent is because - לא על אמונתו הלהו, but one who accepts to be an areiv at the time that the money is lent would be an areiv even if he doesn't use the language of shlichus. The Nesivos concludes that what binds an areiv is the fact that the lender is willing to lend based on his acceptance of responsibility. This is the simple understanding of the gemara - בההוא הנאה דקא מהימן ליה, גמר ומשתעבד נפשיה.

The gemara says in kiddushin that if a woman says to a man, give money so my friend and I will be mikudeshes to you, the kiddushin is binding based on the din areiv. Just as an areiv doesn't receive money yet he binds himself as a result of the pleasure he receives by the lender willing to lend as a result of his acceptance of responsibility, so too the woman receives pleasure that will make her mikudeshes by the man willing to give money based on her word. If we assume like the ketzos that what binds an areiv is the concept of shlichus, how is the woman mikudeshes? We would consider the man to be serving as her agent to give the money to her friend, but how do we consider it as if she herself received the money? This would seem to be a proof to the nesivos that the ability of an areiv to be mi'shabed himself is the pleasure he receives in exchange by his command being followed, not the concept of shlichus.

This also seems to be clear from the gemara's original source - אנכי אערבנו מידי תבקשנו, which is when Yehuda accepted responsibility for Binyamin if he fails to return him back to Yakov. There is no concept of shlichus in that case, rather it was Yehuda willing to be mi'shabed himself as a result of the hana'ah he received from Yakov trusting him.