1) WHAT HAPPENS TO THE ANIMAL WHEN ONE TRANSGRESSES "BAL TE'ACHER"
QUESTIONS: The Gemara derives from the verse, "v'Hayah Becha Chet" -- "It shall be a sin for you" (Devarim 23:22) -- that although one who delays the sacrifice of a Korban transgresses the prohibition of Bal Te'acher, the animal itself does not become disqualified and still may be offered as a Korban. The Gemara notes that this verse seems extra because a different verse teaches that a Korban does not become disqualified when its owner transgresses Bal Te'acher. The verse compares a Bechor to Ma'aser: just as Ma'aser may still be eaten two or three years after it was separated, a Bechor which was not offered within a year does not become disqualified and may still be offered as a Korban.
RASHI explains why one would have thought that a Bechor becomes disqualified after a year. Once a year has passed from the time of the Bechor's birth, it is likely that three Regalim have also passed (and thus the prohibition of Bal Te'acher was transgressed). In his previous comment (DH she'Avrah Shenaso), Rashi explains that the Torah commands that the Bechor be eaten within one year of its birth (as the Mishnah teaches in Bechoros 26b).
There are several questions on Rashi's words.
(a) Rashi says that a Bechor should become disqualified after one year has passed because a year includes three Regalim. However, a year can pass which does not include three Regalim, as the Gemara mentions later (6b; for example, a leap year). Why, then, does the Gemara ask that the verse already taught that a Korban is not disqualified when the owner transgresses Bal Te'acher? Perhaps the verse of Bechor teaches that the Korban is valid even though it was not offered within a year of its birth -- when that year did not contain three Regalim. The verse of "v'Hayah Becha Chet" is necessary to teach that the Korban is valid even when three Regalim have passed. (TUREI EVEN)
(b) Why does Rashi mention altogether the Mitzvah to eat a Bechor within a year of its birth? The Gemara here discusses only whether or not the passage of three Regalim will invalidate a Korban. It does not discuss the Mitzvah to eat a Bechor within its first year, which is a new concept specific to Bechoros. (CHIDUSHIM U'VI'URIM)
(c) The Gemara later (6b) says that the prohibition of Bal Te'acher applies after one year has passed even when three Regalim do not occur in that time (see Rashi there, DH Echad Malei). Why does Rashi here say that it is likely that three Regalim pass within a year? The prohibition of Bal Te'acher applies as long as one year passes, even without three Regalim. (PNEI YEHOSHUA)
(a) The CHESHEK SHLOMO explains that the prohibition against eating a Bechor after one year has passed is derived from the verse which requires one to eat it within its first year. (The Torah requires that a Bechor's blood and Eimurim (fats and certain other parts) be offered on the Mizbe'ach and that its meat be eaten in Yerushalayim (by Kohanim) during the following two days and the intervening night.) The prohibition, therefore, is one of Achilah: one may not eat the Bechor after one year. The Torah does not forbid one from offering the Bechor as a Korban after one year. However, the only way to eat the Bechor is by slaughtering it, and when the animal is slaughtered it must also be offered as a Korban. Therefore, one must bring the Bechor as a Korban within its first year and eat it.
Further proof that the prohibition is an Isur Achilah and is not related to the Bechor's status as a Korban is the Torah's requirement that the Bechor be eaten within the year even when it is blemished and cannot be offered as a Korban.
Since the prohibition is an Isur Achilah, there is no need for a verse to teach that it is still a valid Korban after a year has passed. Nothing improper was done with the Korban that would disqualify it from being offered. The reason why the Gemara assumes that a Bechor becomes disqualified after one year is because of a different prohibition -- that of Bal Te'acher, which is not an Isur Achilah but rather is related to the laws of offering a Korban.
This answers the first question. The verse which compares a Bechor to Ma'aser teaches a detail in the laws of Bal Te'acher: when three Regalim pass the animal remains valid. If this was not the intent of the verse, the verse would not be needed because there would be no reason to assume that a Bechor is disqualified after the passage of a year.
(b) The reason why Rashi mentions the requirement to eat a Bechor within a year of its birth is in order to explain why the Beraisa uses the wording "within its year" ("Toch Shenaso"). The Beraisa uses these words to teach that the Bechor is valid even after the passage of three Regalim. Rashi explains that the Beraisa uses the wording of "within a year" to refer to three Regalim because that is the phrase commonly used with reference to the Halachos of a Bechor. The intention of the Beraisa is to teach that a Bechor is not disqualified when three Regalim have passed within the first year.
(c) Rashi later (6b, DH Man Tana) answers the third question. Rashi says that all of the Tana'im mentioned in the Gemara until that point maintain that the prohibition of Bal Te'acher occurs only when three Regalim pass, but not when one year passes without three Regalim. There is another Tana, who has not yet been quoted, who teaches that even the passage of a year without three Regalim constitutes Bal Te'acher.