Bava Kamma 15B

״המית שורי את פלוני״ או ״שורו של פלוני״⁠ – הרי זה משלם על פי עצמו.

If one admits: My ox killed so-and-so, or my ox killed so-and-so’s ox, then this person pays based on his own admission.

מאי, לאו בתם?

The Talmud analyzes the Mishnah: What, is the halakha of the Mishnah not stated with regard to an innocuous ox? If so, this Mishnah proves that one is required to pay for half the cost of the damage even based on one’s own admission, which demonstrates that the payment is monetary restitution and not a fine.

לא, במועד. אבל תם מאי ⁠– הכי נמי דאין משלם על פי עצמו? אי הכי, אדתני סיפא ״המית שורי את עבדו של פלוני״⁠ – אין משלם על פי עצמו, לפלוג וליתני בדידיה:

The Talmud rejects the proof: No, the halakha of the Mishnah is stated with regard to a forewarned ox. The Talmud asks: But according to this explanation, if it had been an innocuous ox that gored, what would be the halakha? Would one say that indeed, he does not pay based on his own admission? But if so, rather than teaching in the latter clause of the Mishnah in Ketubot: If one admits: My ox killed so-and-so’s Canaanite slave, he does not pay based on his own admission, let the Mishnah differentiate and teach the distinction with regard to the case raised in the first clause itself. The reason for the distinction between the cases when one’s ox kills a Jew or a Canaanite slave is that in the first one pays monetary restitution, while in the second one pays a fine. If the Mishnah wishes to demonstrate the difference between a fine and monetary restitution with regard to admission, then instead of introducing a new case, the Mishnah would have modified the case of the previous clause.