Bava Kamma 43B

מיהו, בן חורין דמשלם כופר על פי עצמו ⁠– והיכי דמי? דאי אתו סהדי ואסהידו ביה דקטל, ולא ידעי אי תם הוה אי מועד הוה, ואמר מריה דמועד הוא, דמשלם כופר על פי עצמו ⁠– היכא דליכא עדים, משלם דמים;

but there is a distinction between the two cases. With regard to an ox killing a freeman, there is a scenario where the owner pays ransom based on his own admission. And what are the circumstances? If witnesses came and testified that the ox killed a freeman and is therefore liable to be killed by stoning, but they did not know whether it was innocuous or if it was forewarned, and its owner said that it was forewarned. In this case, the owner pays ransom based on his own admission, since the ox is liable to be killed, and the ransom is for atonement and does not constitute a fine. Therefore, even where there are no witnesses, and the ox is therefore not liable to be killed by stoning for killing a person, nevertheless, the owner at least pays the monetary value of the victim based on his admission.

גבי עבד, שאינו משלם קנס על פי עצמו ⁠– והיכי דמי? דאי אתו עדים ואסהידו ביה דקטל, ולא ידעי אי תם הוה אי מועד הוה, ואמר מריה מועד הוא ⁠– לא משתלם קנס על פי עצמו. היכא דליכא עדים ⁠– לא משלם דמים.

By contrast, with regard to an ox killing a slave, there is a scenario where one does not pay the fine of 30 shekels based on his own admission. And what are the circumstances where one would be liable to pay this fine due to his own admission? If witnesses came and testified that the ox killed a slave, but they did not know whether it was innocuous or if it was forewarned, and its owner said that it was forewarned. In this case, even though the ox is liable to be killed, the owner does not pay the fine based on his own admission, due to the principle that one does not pay a fine based on his own admission. Therefore, in a case where there are no witnesses, he does not even pay the value of the victim based on his own admission.