Nedarim 43A

גמ׳ בשלמא לא ילונו ⁠— דקא מהני ליה. אלא לא ילוה הימנו, מאי קא מהני ליה? ובשלמא לא ילוה הימנו ולא יקח הימנו ⁠— דקמיתהני מיניה, אלא לא ישאל הימנו, מאי קא מיתהני מיניה!

Talmud: Granted that the person from whom benefit is forbidden may not lend money to the person for whom benefit is forbidden, as he thereby benefits him. However, with regard to the fact that the former may not borrow money from the latter, in what way does he benefit him by borrowing his money? And it could even be said, granted that he may neither borrow money from him nor purchase an item from him, as one benefits in lending money by preserving the value of that money in case the coins deteriorate, and in selling by ridding oneself an item that is difficult to sell. However, with regard to the fact that the person from whom benefit is forbidden may not borrow an item from the person for whom benefit is forbidden, in what way does the lender benefit from him? The borrower returns the same item to the lender.

אמר רבי יוסי ברבי חנינא: כגון שנדרו הנאה זה מזה. אביי אמר: גזירה לשאול משום להשאיל, וכן בכולהו גזירה.

R' Yosei, son of R' Ḥanina, said: The Mishnah is referring to a case where they both vowed that benefit from each other is forbidden. Clearly, then, neither of them may lend to the other or borrow from him. Abaye said: It is possible to explain the Mishnah as it is written, as referring to a case where only one vowed that benefit from the other is forbidden. However, the rabbis issued a decree that it is also prohibited for one to borrow from a person for whom benefit from him is forbidden, due to the concern that he might come to lend to him, as reciprocity is common in these matters. And likewise, that is the explanation in all the cases in the Mishnah; it is prohibited to borrow money, borrow items, and to purchase items from him due to a rabbinic decree, lest he come to benefit him.