Rashi explains that the Sages permitted the original animal whose ear was cut by the Roman official because they assumed that he had no intent to permit the animal, he simply wanted to poke fun at the Jews by performing an action that was forbidden to them. When they later realized that he was doing it to permit the animals, they ruled that the animals could not be slaughtered. A different approach is suggested by the Rambam in his Commentary to the Mishnah. According to the Rambam, the Roman official knew that his actions would permit the animal, but the Sages allowed it since he was not instructed to do this by the Jewish owner. Once he did it many times – and the Jews did not object – the Sages ruled the animal forbidden, since it is as though they had asked him to injure the animal.
כ״ג בכסלו ה׳תשע״ב (December 19, 2011)
Bekhorot 35a-b – Injuring firstborn animals
As we have learned, a firstborn kosher animal must be given to the kohen so that it can be sacrificed in the Temple. After the destruction of the Temple the kohen had to wait until the animal developed a permanent mum – a blemish – at which time the animal became the property of the kohen who was then allowed to eat it like any other meat.
This law creates a situation whereby it is to the advantage of the kohen for the firstborn to develop a mum. The Mishnahon today’s daf (=page) teaches that when there was reason to suspect that a mum was deliberately inflicted on the animal, the Sages forbade the animal from being slaughtered. The Mishnah relates:
It happened that a Roman official saw an old male lamb with its long wool hanging down and asked: what is the meaning of this? —they replied: ‘it is a firstling and is not to be slaughtered until it has a blemish’. The Roman took a dagger and slit its ear. The matter came before the Sages and they permitted it. After they had permitted, he went and cut into the ears of other firstlings. The Sages thereupon forbade them.
The Mishnah concludes:
This is the rule: wherever the blemish is caused with the knowledge and consent of the owner, it is forbidden, but, if it is not with his knowledge and consent, it is permitted.
Two approaches are offered to explain the case of the Mishnah.