Ketubot 81B

ואי סלקא דעתין נתנה כתובה לגבות מחיים, נייחד לה שיעור כתובה, והשאר ליזבין! ולטעמיך, ולותבה ממתניתין: לא יאמר לה הרי כתובתיך מונחת ליך על השלחן, אלא כל נכסיו אחראין לכתובתה?!

And if it enters our mind that a marriage contract can be collected during his lifetime, why is all this necessary? Let him set aside for her part of the property that corresponds to the amount of the marriage contract, and the rest let him sell. Abaye replied: And according to your reasoning, rather than asking this question based on a baraita, let him raise this difficulty from the Mishnah, which teaches that he may not say to her: Your marriage contract is placed on the table for you. Rather, all his property is mortgaged for her marriage contract. Why can’t he designate property equivalent to the sum of her marriage contract and sell the rest?

התם עצה טובה קא משמע לן. דאי לא תימא הכי, סיפא דקתני: וכן לא יאמר אדם לאשתו ״הרי כתובתיך מונחת ליך על השלחן״, אלא כל נכסיו אחראין לכתובת אשתו ⁠— אי בעי ליה לזבוני, הכא נמי דלא מצי מזבין?! אלא עצה טובה קמשמע לן, הכא נמי עצה טובה קא משמע לן,

The Talmud answers: The tanna in the Mishnah there teaches us good advice, i.e., that one should not do so ab initio, so as to ensure that the amount set aside for her marriage contract is not lost, which would necessitate writing a new marriage contract. However, it should not be inferred from the Mishnah that it is prohibited to do so. As, if you do not say so, that it is merely good advice, consider the latter clause of the Mishnah, which teaches: And similarly, a man may not say to his wife: Your marriage contract is placed on the table. Rather, all his property is mortgaged for her marriage contract. If he wants to sell, here too, may he not sell? Rather, in that case the tanna teaches us good advice, and therefore here too, with regard to a yevama, he teaches us good advice.