Gemara

(Beraisa - Rebbi) Question: What do we learn from "va'Ydaber Moshe Es Mo'adei Hash-m"?

Answer: "B'Mo'ado" regarding the Tamid and Korban Pesach teaches that they may be offered on Shabbos and b'Tum'ah;

Had it only taught about Pesach, we would say that this is because there is Kares [for not bringing it], but the Tamid would not be Docheh;

65b (Mishnah - R. Eliezer): Shechitah [of Korban Pesach] is an Av Melachah, yet it is Docheh Shabbos. Haza'ah (sprinkling Mei Chatas to be Metaher from Tum'as Mes) is only Shevus, all the more so it is Docheh!

R. Akiva: We can learn oppositely! Haza'ah is only Shevus, yet it is not Docheh Shabbos. Shechitah is a Melachah, all the more so it is not Docheh!

R. Eliezer: You uproot "b'Mo'ado" - whether Erev Pesach is on a weekday or Shabbos!

R. Akiva: ("B'Mo'ado" refers to the fixed time for Shechitah.) It does not say "b'Mo'ado" regarding these [Shevusim. They could have been done before Shabbos!]

Kesuvos 51b (Shmuel's father): If a Yisrael's wife was raped, she is forbidden to her husband. We are concerned lest initially she was forced, but later desired the Bi'ah.

Question: The Torah permits a raped woman to her husband. According to Shmuel's father, what is the case?

(Beraisa #1): If "she was not grabbed" she is forbidden. Had she been grabbed, she would be permitted;

Rishonim

Tosfos (Pesachim 50a DH Makom): The Yerushalmi explains that we forbid Melachah on Erev Pesach, for on this day we offer Pesach. We permit in the morning, for one may not offer Pesach then. Melachah would be forbidden every day, due to the Tamid, if not for "v'Asafta Deganecha." It connotes that the Torah forbids.

Chasam Sofer (YD 109): Tosfos connotes unlike the Taz (brought below, who says that Chachamim cannot forbid what the Torah explicitly permits).

Note: Perhaps this is because the Torah permits before midday, and Chachamim forbid where the custom is to forbid. Alternatively, Chachamim forbade other days, e.g Purim, a Chasan, an Avel, Yom Tov Sheni...

Tosfos (Chulin 12a DH Pesach): The Gemara did not ask if R. Meir could be stringent to forbid eating meat not for a Mitzvah. R. Meir forbids only mid'Rabanan. We ask that he cannot be stringent about Pesach and Kodshim. Just like R. Meir distinguishes between what is possible [to be stringent about] and what is impossible, also Chachamim.

Tosfos (Kesuvos 51b DH Ones) says that this is a Torah concern, for presumably Chachamim would not make her husband divorce her due to this.

Birkei Yosef (588:2): Tosfos could agree to the Taz. Perhaps "concerned" connotes that the Torah was stringent. A Beraisa (Sukah 43a) expounds that we take the Lulav "ba'Yom ha'Rishon" - even on Shabbos. According to the Taz, how can the Tana explain why Chachamim decreed against this?

Note: It is not an Asmachta, for the Gemara asked why we need a verse to permit Muktzeh, and answered that the Tana permits Machshirav (e.g. to detach it on Shabbos). Perhaps we can answer that Chachamim did not forbid everywhere, e.g. in Har ha'Bayis (42b).

Poskim

Shulchan Aruch (OC 588:5): When Rosh Hashanah falls on Shabbos, we do not blow the Shofar.

Taz (5): This is a decree lest one carry a Shofar four Amos in Reshus ha'Rabim. The Mizrachi asked why we don't decree every year, lest one fix a Kli Shir! I answer that this does not apply to a Shofar, which is not for music, rather, for a Mitzvah. Further, Chachamim should not totally uproot the Torah's command. The decree on Shabbos does not totally uproot the Mitzvah. Chachamim did not decree against Bris Milah on Shabbos [lest one carry the baby in Reshus ha'Rabim], for this reason. It says "ba'Yom ha'Shemini", [which we expound to teach] even on Shabbos.

Birkei Yosef (2): If a married woman was raped, Shmuel's father is concerned lest she consented at the end. The Gemara asked in what case the Torah permitted! Tosfos (51b) says that Chachamim would not make her husband divorce her due to this. This connotes that for a better reason to separate them, they would decree, even though the Torah explicitly permits Ones to a Yisrael. The Rambam holds that the Torah is lenient about Safek, and Chachamim were stringent. We must say that Chachamim were stringent about the Safek. What was the question "in what case did the Torah permit?" This is like the Taz, that where the Torah permits, Chachamim cannot forbid. However, we could say that here the Torah is stringent, for in most cases she consents at the end, like the Yerushalmi says.

Chavos Ya'ir (142): The Taz' rule is not always true. The Torah forbids fruits of a tree the first three years, and in the fourth year "Kol Piryo Kodesh Hilulim la'Shem" (Vayikra 19:23-24). There is a Tana who taught "Kerem Revai", i.e. Revai applies only to vines, but not to other trees, even mid'Rabanan, even though all agree that Orlah applies to all trees. This is unlike Ma'aser: Even though most Poskim say that the Torah obligates only grain, wine [and oil], they agree that mid'Rabanan, all species are obligated. However, before they decreed, how could Chachamim exempt what the Torah obligated ("Kol Tevu'as Zar'echa")?

Chasam Sofer (YD 106): The Chavos Ya'ir challenged the Taz because Chachamim expounded unlike the simple meaning of the verse. Surely the Taz agrees that Chachamim may add or subtract from the verse according to their tradition! The Taz means only that Chachamim would not enact unlike the meaning of the verse.

Note: One opinion in Tosfos (Kidushin 2b DH Esrog) that the Tana who taught Kerem Revai obligates trees mid'Rabanan. However, this is not difficult for the Taz, for the Torah exemption is not explicit. Chavos Ya'ir questioned how the Taz can explain why Chachamim could expound to exempt!

Tzlach (66a DH B'Mishnah): How can it be that R. Akiva did not know the Drashah "b'Mo'ado"? This was known from Hillel! We must say that he knew it, but held that Chachamim decreed to forbid Haza'ah, even though it will be Mevatel people from Korban Pesach. All the more so, they should have decreed to forbid Shechitah, which is a Melachah! R. Eliezer challenged this, like the Taz, that Chachamim cannot forbid what the Torah explicitly permitted.

Tzlach (70b DH uv'Yaldusai): If "b'Mo'ado"connotes [even on] Shabbos and b'Tum'ah, we must say that there is no Hekesh equating Chagigah of the 14th to Pesach regarding these. If there were, why does it say b'Mo'ado regarding Tamid and other Korbanos Tzibur? We could learn them from Pesach! Even though we could ask "you cannot learn from Pesach, for there is Kares [for one who does not bring it]" (77a), we could learn from Chagigah of the 14th, for which there is no Kares. However, if "b'Mo'ado" does not connote Shabbos and b'Tum'ah, rather, we learn from a Gezeirah Shavah, we can learn Chagigah of the 14th from a Hekesh.

Minchas Yitzchak (8:38): The Taz wrote that if we expound the connotation of b'Mo'ado" to include Shabbos and Tum'ah, it would be an Akirah for Rabanan to forbid [Korban Pesach on Shabbos]. If we learn only from a Gezeirah Shavah, [this is not considered explicit in the Torah]. The Rambam (Hilchos Ishus 1:1) says that Kidushei Kesef is mi'Divrei Sofrim, even though it is learned from a Gezeirah Shavah. He holds that such matters are not called explicit in the Torah. If so, this is not related to the Taz' Chidush.

Pnei Yehoshua (Kidushin 73a DH veha'Torah): Why did the Gemara ask why Chachamim forbade a Shtuki (a Safek Mamzer)? In many places Chachamim were stringent about a common minority. All the more so, they should be stringent about an even Safek! I answer that since the Torah explicitly permitted this Safek, we cannot be stringent even mid'Rabanan. This reasoning occurs in several places, e.g. Kidushin 10b.

Note: Ravina explains that Rebbi Yehudah ben Beseira holds that since a Kal va'Chomer permits a Bas Yisrael Mekudeshes to a Kohen through money to eat Terumah, Chachamim would not decree to forbid her lest she has a blemish that invalidates the Kidushin. However, they decreed for another reason (lest she feed Terumah to her siblings). The Pnei Yehoshua calls what is learned from a Kal va'Chomer "explicit in the Torah."

Pnei Yehoshua (ibid.): The Rambam and his followers learn from Safek Mamzer that the Torah permits an even Safek, and Chachamim forbid. If not for the desire to be stringent about lineage, we would not be stringent where the Torah explicitly was lenient. This requires investigation.

Chasam Sofer (YD 73 DH veha'Taz): Even though the Torah explicitly permits fowl with milk ("Gedi", and not fowl), this refers to cooking. Chachamim could not forbid this. They decreed only against eating them. The Rambam teaches that Chachamim may not make a fence to forbid what the Torah explicitly permits, due to Bal Tosif and Bal Tigra.