Gittin 50A

קבלן דבעל חוב ⁠– דברי הכל משתעבד; ערב דבעל חוב, וקבלן דכתובה ⁠– פליגי; איכא למאן דאמר: אף על גב דלית ליה נכסי ללוה ⁠– משתעבד, ואיכא למאן דאמר: אי אית ליה ⁠– משתעבד, אי לית ליה ⁠– לא משתעבד.

Everyone also agrees that an unconditional guarantor for the repayment of a creditor becomes responsible for payment of the borrower’s debt. The rabbis disagree with regard to the following cases: A guarantor for the repayment of a creditor and an unconditional guarantor for payment of a marriage contract. There is one Sage who says: Although the borrower does not have property of his own at the time of the loan, the guarantor becomes responsible for payment of the obligation. And there is one Sage who says: If the obligated party has property of his own, then the guarantor becomes responsible; if he does not have property of his own, he does not become responsible.

והילכתא: בכולהו, אף על גב דלית ליה ⁠– משתעבד; בר מערב דכתובה, דאף על גב דאית ליה ⁠– לא משתעבד.

And the halakha is: In all of these cases, although the obligated party does not have property of his own at the time he assumes the obligation, the guarantor becomes responsible for payment of that obligation. This is so except in the case of a guarantor for payment of a marriage contract, as in that case, even if the husband has property of his own when he gives a marriage contract, the guarantor does not become responsible for payment of the obligation.

מאי טעמא? מצוה הוא דעבד, ולא מידי חסרה.

What is the reason for this? The intention of the guarantor is performing a mitzva of facilitating a marriage by encouraging the woman to consent to the marriage as a result of his involvement, but he does not truly intend to obligate himself. And furthermore, the woman does not lose anything in exchange for which the guarantor would accept responsibility, as the husband does not borrow money from the woman. Therefore, the rabbis instituted that the guarantor does not become responsible for payment of the marriage contract from his own property.