Gittin 14A

בההיא הנאה דקא משתניא ליה בין מלוה ישנה למלוה חדשה, גמר ומשעביד נפשיה.

With that benefit that the borrower receives when his debt changes from an old debt to a new debt, he fully pledges himself to the new lender. The borrower prefers to have a new lender from whom he can ask for an extension of the deadline for the repayment of his debt.

אמר ליה הונא מר בריה דרב נחמיה לרב אשי: אלא מעתה, כגון הני דבי בר אלישיב, דכפתי ושקלי לאלתר, הכי נמי דלא קנו?!

Huna Mar, son of Rav Neḥemya, said to Rav Ashi: If that is so, what about a case where the debt is transferred to these members of the house of bar Elyashiv? As, those implacable people bind debtors with handcuffs and take their money immediately when the time for repayment arrives. They certainly would not add to the time that the borrower has to repay the first loan, and he would not willingly pledge himself to them. So too, will you say that they do not acquire the money in the presence of all 3 parties, as the borrower would not agree to this arrangement?

וכי תימא הכי נמי, אם כן נתת דבריך לשיעורין!

And if you would say that indeed this method is not effective in a case of this kind, if so you have rendered your statement subject to circumstances. In other words, the application of the acquisition in the presence of all 3 parties is not absolute, and instead depends on whether in this particular case the borrower agrees to transfer his debt from one person to another.

אלא אמר מר זוטרא: הני תלת מילי שוינהו רבנן כהלכתא בלא טעמא: חדא ⁠– הא; ואידך ⁠– דאמר רב יהודה אמר שמואל: הכותב כל נכסיו לאשתו ⁠– לא עשאה אלא אפיטרופיא; ואידך ⁠– דאמר רב חנניא: המשיא אשה לבנו גדול בבית ⁠– קנאו.

Rather, the Talmud retracts the previous explanation in favor of that which Mar Zutra said: These 3 matters were instituted by the rabbis as a halakha without a reason, i.e., they instituted these ordinances despite the fact that the logical mechanism by which they function is unclear: One is this case of an acquisition in the presence of all 3 parties. And another is that which Rav Yehuda says that Shmuel says: One who writes over all of his property to his wife as a gift has rendered her only a steward [eppitrofeya], but she does not become the owner. And another is that which Rav Ḥananya says: With regard to one who marries a woman to his eldest son, and the wedding takes place in his house, the son has acquired that house.