פירוש על בבא קמא 6:3

One who stacks in the midst of a field of his friend without permission etc....

ואם הדגיש ברשות בעל השדה חייב – In the Gemara (Tractate Bava Kamma 59b), it establishes it in a valley, for everyone together are accustomed to make it in one garden bed. This is his heap of sheaves and that is the other’s heap of sheaves and they appoint a guard. And since the guard state that he will go up and make a heap of sheaves, it is as if he said: “I will go up and guard yours.” But with the rest of the people, even if he made a heap of sheaves with permission, the owner of the field is not liable until he accepts upon himself protection/guardianship.

Three
If a man stacked his sheaves in his fellow’s field without his permission, and the owner of the field’s beast ate the sheaves, he is exempt. If [the beast] was injured by them, the owner of the stack is liable. If he made the stack with his permission, the owner of the field is liable. This mishnah contains laws that were all learned previously in chapter five mishnah three. We will just summarize. If someone brings his belongings onto another’s property without permission and his belongings are damaged there, the property owner is not liable. If the belongings damage the other person’s property he will be liable. However, the reverse will be true if he brings the belongings with the other person’s permission.

Since we have said the guardian of the field in a valley and to any person they can stack in the midst of the field, and he gives him permission... Behold it is as if he has accepted upon himself to guard it. And therefore the guard will be obligated. And even though he did not tell him 'I will guard it'... However the owner of a house we have explained that he is not obligated until he accepts upon himself to guard it.