[1) The Rashba was asked a reverse sort of question in Teshuvas HaRashba 1:326; cited in Maharitz Chiyes, at the back of the Gemara here. The question was how is it possible to make a blemish in the film of the eye? The questioner suggested that what actually happened was that bar Kamtza switched the animal given to him by the king, with another animal which already had a "Dok" in the eye. I do not think this is so relevant to your question, so I am not going further with this.]

2) However, Maharitz Chiyes then cites something which is more relevant; the Midrash Rabah Eichah on the verse (Eichah 4:2) "The precious children of Tzion". This is in Parshah 4:3.

Bar Kamtza said to the governor that all the sacrifices that you send to the Jews are eaten by them and they sacrifice other animals instead. He said that if you do not believe me then send a prefect (??????) with me and the sacrifices, and you will know immediately that I am not lying. They went on the journey and the prefect fell asleep. Bar Kamtza got up in the middle of the night and cast blemishes in hidden places in the animals. The Cohen in the Beis Hamikdash noticed these blemishes, and sacrificed other animals in place of them. The Roman officer eventually found out that the Cohen had not sacrificed the animals that the governor sent.

3) The account related by the Midrash is not the same as the account in our Gemara but I think that we learn from this Midrash that it is probable that the Romans kept track of the animals they were sending to Yerushalayim. Therefore if they would find out that their animal has been rejected as a korban; delegated to pasture, and a different animal selected as a korban; this could be very dangerous. It might be more dangerous than what they actually did; namely reject the korban and bring nothing in its place. That way they did not actually infer that the Jewish korban was better than the Romans, but if they would reject the Roman animal and offer another in its place this would be a terrible insult.

Dovid Bloom

Further comment:

Of course, one does not have to say that the Romans necesarily kept track of the korbanos, because even if they did not, Kamtza might go back to Rome and report that the Jews have replaced your korban with their own Korban. The Romans would not be happy to hear that their animal; which they hoped was going to be a beautiful korban in the Holiest place on earth, was now pasturing. If they wanted their animals to pature in luxury, they did not have to send them to Eretz Yisrael to do that; they could do that in Italy.

Dovid Bloom

Support for above from Mefarshim:-

I found, bs'd, that I was Mekaven to one or two things that the Achronim wrote here.

1) Sefer Geresh Yerachim, by the Maharal Tzintz, writes that the Sages of that time knew that Bar Kamzta was determined at all costs to take revenge against the Jewish People and would stop nowhere in his fanatic mission. Even if the sacrifice would have been offered up properly he would not have given up, but would have thought of new ways of making trouble.

According to this it is easier to appreciate that if another animal would have been brought from the Bedek HaBayis, Bar Kamtza would have made sure that he knew about this very soon, and would have now a new reason to say Loshan Hora to the king and make things even worse.

2) I also found that Ben Yehoyada, by the Ben Ish Chai, wrote that certainly the king sent his servants to accompany Bar Kamtza to Yerushalayim, since he would not rely on Bar Kamtza alone. This supports, bs'd, what I wrote in my first reply that the Romans kept track of the animals they sent. In fact, there is a support for Ben Yehoyada in the Midrash that I cited; in section 2); that the governor sent a prefect to ensure that Kamtza was not lying. Since the king sent his officers, it would be very difficult to bring another animal in place of the one that Kamtza brought.

Yasher Koach Gadol

Dovid Bloom

The words of the Chasam Sofer:-

There is an explanation here, in Chidushei Chasam Sofer DH R' Zechariah, which I thinks sheds light on the whole tragic episode and also can help us to answer your question, David.

1) The Chasam Sofer writes that he does not want us to get the impression that R' Zecharia ben Avkulas, who was a Tzadik, was responsible for the destruction of the Beis Hamikdash. The point is that there had never; in the history of the Jewish People; been such a malicious individual as Bar Kamtza, who for such a trivial reason was prepared to hand over Klal Yisrael and the Beis Hamikdash into the hands of the nations and R' Zecharia could not have been expected to aniticipate this. Chasam Sofer cites the Gemara in Maseches Tamid 32a which states "Who is a wise man? The one who sees the 'nolad' (i.e. who has foresight)". It does not say "The one who sees the future to come". The Chacham is not expected to predict the future. Mishlei 22:13 states "The lazy man says 'there is a lion outside; I will be slain in the streets!'". The lazy man invents imaginary dangers which never happened. This is why the wise man is not somebody who foresees, since any fool can be afraid of all sorts of nonsense. The wise man is one who learns from past experience and, on the basis of this, judges what might happen in the future. So R' Zecharia was not to blame for what happened because it was totally unpredictable according to logic.

2) Chasam Sofer notes the verse with which Rabbi Yochanan commenced the Gemara in 55b "Happy is the man who is always anxiuos" (Mishlei 28:14). At first sight this seems to contradict directly Mishlei 22:13 which condems the lazy man's fear of the lion. The answer is that 22:13 refers to where we have no past experience of lions in the street, and it is all in the sluggard's imagination. 28:14 discusses what we have to do if such a thing has already happened. It teaches that from now on we have to be worried about such low individuals as Bar Kamtza and the likes of them.

3) There is a common factor with the Geresh Yerachim, that I cited above, and the Chasam Sofer. Geresh Yerachim writes that Bar Kamtza recoginized no limits for what he is prepared to do for his slander to the king. Chasam Sofer writes that nobody had ever gone so low as Bar Kamtza. So tricks like switching the korban would not help when there were such disgusting people amongst us.

Dovid Bloom