Commentaire sur Bava Kamma 5:3

A person brought in an ox to a courtyard of a homeowner without permission etc...

היה אביו או בנו – of the owner of the house were in it (i.e., the cistern) and the same law applies for the rest of the people, but [the Mishnah] took a usual incident.

If a man brought his ox into the courtyard of a householder without permission and the householder’s ox gored it or the householder’s dog bit it, the householder is not liable. If [the first man’s ox] fell into [the householder’s] cistern and polluted its water, he is liable. If [the householder’s] father or son was in [the cistern and it killed them] the ox’s owner must pay the ransom price. But if he had brought his ox in by permission the owner of the courtyard is liable.
Rabbi says: “In no case is [the householder] liable unless he had agreed to watch over it.
The first two clauses of mishnah three are similar to what we learned in the previous mishnah. We again learn that if a person brings his belongings onto another person’s property without permission and they are damaged he will not receive compensation and if they cause damages he is liable. In clause b we learn an additional law. If when falling into the cistern the ox kills a person, the owner of the ox is obligated to pay the ransom price mentioned in Exodus 21:30. As we learned in chapter four mishnah 5 when an ox kills a person, the owner of the ox is obligated to pay a ransom price to the family of the deceased. In our mishnah we learn that falling into a cistern is similar enough to goring that the ox’s owner is obligated.
At the end of this mishnah we learn that Rabbi (Rabbi Judah Hanasi) holds an opinion different from that held in the anonymous previous portions of the mishnah. According to Rabbi, allowing a person to enter one’s property is not tantamount to accepting upon oneself the obligation to watch over that person’s belongings. When I allow you to bring your belongings onto my property, I may still assume that you will watch over your things. Therefore, if they cause damage you are still obligated and if they are damaged I am exempt. According to Rabbi, a person does not accept the responsibility for watching the belongings until he states so explicitly. The other opinion in the mishnah held that by allowing a person to enter one’s property on is implicitly accepting upon himself the responsibility for that person’s belongings.

this is the case where that ox was 'accustomed' to fall on people in pits, therefore he's obligated to pay the ransom, specifically when he makes himself fall there because of food that he saw in the pit, and therefore the ox is not obligated to be killed. And the halakah is like Rebbi that he says that the owner of the house is not obligated in anything if he did not accept upon himself to guard it, and similarly the owner of the ox is not obligated in anything since he had permission to enter.

משלם את הכופר – as for example, the [owner of the] animal had been forewarned to cause himself to fall upon on people in cisterns and currently saw herbs in the pit and caused itself to fall into the cistern to eat the herbs and killed a person, that the [owner of the] ox is exempt from the death penalty, for it killed without intention. But the owners [of the ox] pay the ransom, for it includes the ransom even if the animal killed without intention, as we have stated above.

רבי אומר וכו' – And the Halakha is according to Rabbi [Yehuda HaNasi]. Therefore, if [the owner of] the animal led his ox [into the courtyard] with the permission of the owner of house, in an undefined manner, and he didn’t accept upon himself to guard [the animal], the owner of the house is exempt, for he did not accept upon himself watching/proper care. And the one who brings it (i.e., the animal) in is also exempt [from liability] because he brought it (i.e., the animal) in with permission.