Bava Batra 176 - How to Become Wise
If the pledge of a guarantor appears after the signatures on the document, the lender can collect only from the guarantor's unencumbered properties, but not from properties that the guarantor has since sold to others. Since the guarantee is not attested to by witnesses, it has no more power than an oral obligation.
Ben Nannas, however, says that in this case the lender cannot collect from the guarantor at all. Said Rabbi Ishmael to him, "Why?" Explains Ben Nannas: If a creditor was strangling someone in the street, and this fellow encountered them and said to the creditor, "Leave him be, and I will give you the money that he owes" - the guarantor is not liable, because the lender did not lend the money trusting the guarantor in the first place.
Said Rabbi Ishmael, "One who wishes to become wise should occupy himself with the study of monetary law", for no other branch of Torah law provides the human intellect with as wide-ranging a field of study for reasoning and analysis. Continued Rabbi Ishmael, "And one who wishes to occupy himself with the study of monetary law should serve Shimon ben Nannas." Nevertheless, the law follows Rabbi Ishmael.
Art: Pieter Codde - Young Scholar in His Study