Gittin 20A

אי משום כריתות דאית בה ⁠– הא בעינא ״וכתב לה״⁠ – לשמה, וליכא.

If there is a concern that the Torah scroll can effect a divorce due to the verses concerning severance of marriage that there are in it, as it is written: “And he writes her a scroll of severance” (Deuteronomy 24:1), this is also not problematic, since it is required, as it states: “And he writes her.” This indicates that the bill of divorce must be written for her sake, i.e., it must be written for the express intent of being used to effect divorce between this specific man and this specific woman, and this is not so in the case of a Torah scroll.

וכי תימא, ליחוש דילמא אקדים ויהב ליה זוזא לספרא מעיקרא; הא בעינן ״שינה שמו ושמה, שם עירו ושם עירה״, וליכא.

And if you would say that there is reason to be concerned that perhaps he first gave a dinar to the scribe at the outset, when he wrote the Torah scroll, and told him to write the verses discussing divorce for his wife’s sake, isn’t there a need for other things to be written in a bill of divorce as well, as the Mishnah (80a) teaches: If the scribe changed his name or her name, the name of his city or the name of her city, then the bill of divorce is invalid. The validity of a bill of divorce depends upon these details being written, and they are not in the Torah scroll.

ורב יוסף מאי קא משמע לן? שאין מי מילין על גבי מי מילין.

The Talmud asks: But if so, what is Rav Yosef teaching us with his statement? It seems obvious that there is no reason why a Torah scroll could be used as a bill of divorce. The Talmud says: He is teaching us that gall water is not permanent when applied on top of gall water. The novel idea is that there is no need to be concerned that the scribe wrote a bill of divorce in gall water on the outside of the Torah scroll.