What is the significance of a Chasar Lev shaking hands?
Rashi #1: He accepts to be a monetary guarantor.
Rashi #2: He shakes hands with heresy, to go in its ways.
R. Yonah: He is called Chasar Lev, for he brings himself to quarrels with the creditor, who will ask him to pay if the borrower lacks the money to pay. Normally, an Arev does not pay willingly, only via coercion of Beis Din, for he did not benefit [from the loan]; the borrower should strive to pay. Also afterwards, he will quarrel with the borrower to pay [what he paid on behalf of him] when the latter has property. Feuding is lowly - "Ohev Pesha Ohev Matzah" (19).
Malbim: Even though one must do everything for his friend, he need not enter Arvus and shaking hands for him. If he does so, he is Chasar Lev and does not see the future.
Why does it say "Orev Arubah Lifnei Re'ehu"?
Rashi: He already accepted Arvus to Hashem, to guard his Mitzvos 1 .
R. Yonah: One who accepts to be an Arev, he is negligent about guarding his money. Tzadikim's money is dearer to them than their bodies (Chulin 91a), lest [amidst need] they come to steal. He is ashamed to refuse [to accept Arvus] in front of Re'ehu (the borrower). It is improper to be ashamed to refuse doing what Da'as obligates to refuse! He should calculate the loss compared to the reward. It is better to refuse and transgress the borrower's desire now, than the quarrel and hatred that he will find at the end, if the borrower will not pay.
