Ketubot 102A

לא, בשטרי פסיקתא, וכדרב גידל.

The Talmud rejects this: No, the Mishnah is referring to a case of documents of stipulation that record the amounts that parents agree to provide to their son or daughter, and this is in accordance with the opinion of Rav Giddel.

דאמר רב גידל אמר רב: כמה אתה נותן לבנך? כך וכך. וכמה אתה נותן לבתך? כך וכך. עמדו וקידשו ⁠— קנו. הן הן הדברים הנקנין באמירה.

As Rav Giddel said that Rav said: When two families negotiate the terms of marriage for their respective children, one side says to the other: How much do you give your son? And the second side answers: Such and such amount. How much do you give your daughter? And the first side responds: Such and such amount. Then, if the son and daughter arose and performed the betrothal, all of these obligations are acquired and therefore binding. These are among the things that are acquired through words alone, without the need for an additional act of acquisition. The Mishnah is referring to a document that records such an agreement.

תא שמע, כתב לכהן: ״שאני חייב לך חמש סלעים״⁠ — חייב ליתן לו חמש סלעים, ובנו אינו פדוי.

Come and hear another challenge to the opinion of Reish Lakish, based upon the following Mishnah (Bekhorot 51a): If he wrote to a priest with whom he wants to perform the redemption of his firstborn son: I am obligated to pay you 5 sela, then he is obligated to give him 5 sela and his son is not redeemed even once he pays the money. This baraita apparently supports the opinion of R' Yoḥanan.

שאני התם דמשועבד ליה מדאורייתא: אי הכי, אמאי כתב? כדי לברר לו כהן.

The Talmud answers: It is different there, because he is obligated to give the 5 sela to him by Torah law in order to fulfill his obligation of redeeming his firstborn son, even without writing a contract. The Talmud asks: If that is so, why did he write the contract at all? The Talmud answers: In order to select for himself a specific priest with whom to perform the redemption of his son.