The Rambam (Hilchos Issurei Biah 21:32) writes:ולעולם ימכור אדם כל מה שיש לו וישא בת תלמיד חכם שאם מת או גולה בניו תלמידי חכמים, וכן ישיא בתו לתלמיד חכם שאין דבר מגונה ולא מריבה בביתו של תלמיד חכם – “A person should sell all he has to be able to marry off his child to the daughter of a talmid chocham, as if he dies or goes into exile, at least his sons will remain talmiday chachomim. Similarly, one should marry off his daughter to a talmid chocham, as there are no despicable things or quarrels in the house of a talmid chocham.” The Rambam only mentions selling everything one has, however, he doesn’t write that one should take out a loan or go knocking on doors to collect money.
I heard in the name of R’ Nossan Kopshitz shlita that there is no obligation to borrow money, however, it is ‘permissible’ to do so, provided one won’t come to the situation of: לוה רשע ולא ישלם - “A wicked person borrows and doesn’t pay back”, i.e. he has a plan of how to be able to pay back.
I also heard in the name of R’ Chaim Aharon Monderer shlita and R’ Meir Steinberg (a Dayan on the Beis Din HaYashar VeHatov) that the obligation to marry off a son is a mitzvah like all other mitzvos in the Torah, and strictly speaking when it comes to mitzvos there is no obligation to borrow money to be able to fulfill the mitzvah. We don’t find in halachah any precedence that one must borrow money; we do find that one should go knocking on doors, but we don’t find that one should borrow. Nonetheless, it is ‘permissible’ to do so, and it is not a problem of לוה רשע ולא ישלם , as we hear and see stories daily of people doing so and managing to pay back. However, there is no ‘obligation’ to do so.
Moreover, even when it comes to paying off debts, strictly speaking there is no obligation to borrow money to be able to do so, and one can say to the lender, that when I have the money, I will pay back. However, this was in previous generations, however, nowadays, it could be that one is obligated to borrow in order to pay back, as now it is normal to run from one gemach to the next, therefore, it could be that the lender lent the money with the intention, that when the pay-day arrives, the borrower should take money out of a different gemach to pay up the debt.
However, R’ Gershon Meltzer shlita said in a shiur: Nowadays, borrowing money is included in the obligation of marrying off a child, as if one doesn’t have money, this is the only way to do it. The amount one should be borrow, is the amount that Bnei Torah in a similar situation would obligate themselves to give.
