Gemara

Question (R. Yirmeyah): If Kivsei Atzeres were slaughtered Lishmah and the bread was lost (the lambs are Pesulim for Kivsei Atzeres, for Shechitah was Zokek), may Zerikah be done Lo Lishmah (l'Shem Shelamim) to permit the meat?

Answer (R. Zeira): No. We never find that something is Pasul Lishmah and Kosher Lo Lishmah.

(R. Chanina of Tirasa - Beraisa): If four lambs were slaughtered for two loaves, we throw the blood of two lambs Lo Lishmah (l'Shem Shelamim. This permits Haktaras Eimurim and eating the meat.) If we would do Zerikah Lishmah, the latter lambs must be burned.

Question (R. Yochanan): Do we transgress (throw the blood Lo Lishmah) in order to gain (salvage the other lambs)?!

(Mishnah - R. Eliezer): If Ivrei (limbs of) a Chatas were mixed with Ivrei Olah, we burn all of them on the Mizbe'ach.

Chachamim say, we wait for Ibur Tzurah, then all are burned in Beis ha'Sereifah (a place outside the Azarah for burning Pasul Kodshim).

We do not transgress (to be Maktir all the limbs, including Ivrei Chatas) in order to gain (to be Maktir Ivrei Olah)!

Answer: We transgress in (a matter in order to gain in the same matter, e.g. in) a Chatas in order to gain in a Chatas. We do not transgress in a Chatas in order to gain in an Olah.

Question (Beraisa): If Shavu'os was on Shabbos, and Kivsei Atzeres were slaughtered Lo Lishmah, we are not Zorek the blood. If Zerikah was done, this permits Haktaras ha'Chelev at night.

We do not transgress (do Zerikah l'Chatchilah) to gain in the same matter

Answer: We would transgress on Shabbos to gain on Shabbos. Here, the gain is on Motzei Shabbos (we burn the Chelev and eat the meat), so it is forbidden.

Question: We do transgress in a matter in order to gain in a different matter!

(Mishnah - R. Eliezer): If Terumah wine is about to fall into Tamei Chulin below, and one cannot save a Revi'is in Taharah, he lest the Terumah fall (and be Metamei and forbid the Chulin). He may not actively be Metamei it (e.g. to catch it in a Tamei Kli);

R. Yehoshua says, (in order to save the Chulin) he may actively be Metamei the Terumah.

Answer: There is different, for the Terumah will become Tamei in any event.

Gitin 38b (Rav Yehudah): One who frees a slave transgresses an Aseh - "forever you will make them work for you."

41a (Mishnah - Beis Shamai): A half-slave cannot marry a Shifchah, nor a Bas Yisrael. "Lasheves Yetzarah" (the world was created for procreation)! Rather, for Tikun ha'Olam, we force his master to free him, and the half-slave writes a document obligating himself to pay half his value to his master.

Beis Hillel agreed to Beis Shamai.

Pesachim 59a: A Mechusar Kipurim must bring a Korban before eating Kodshim. If he did not do so before the afternoon Tamid on Erev Pesach, to enable him to fulfill Korban Pesach (which has Kares for one who neglects it), we override the Aseh of Hashlamah (not to offer after the afternoon Tamid), which does not have Kares.

Chagigah 9a (Rav Yitzchak): If one transgresses in private, it is as if he repels the feet of the Shechinah.

Question: R. Ila'a ha'Zaken taught that if one sees that his Yetzer Ra is overpowering him, he should go to a place where he is not known, wear black clothes, and do what his heart desires!

Answer: Rav Yitzchak discusses one who could overcome his Yetzer Ra. R. Ila'a discusses one who could not overcome his Yetzer.

Rishonim

Rif (Mo'ed Katan 9a): The Halachah does not follow R. Ila'a. If one's Yetzer is overpowering him, he must settle himself (and overcome it). Everything is in the hands of Shamayim, except for Yir'as Shamayim.

Tosfos (48a DH Chet (1)): One may not transgress the Lav of burning Shirayim of a Chatas for the sake of an Olah, but it is permitted for the sake of a Chatas. We do not transgress breaking a bone of Pesach in order to merit eating the marrow inside (Pesachim 85a), for eating does not fix the Korban as much as burning the limbs (of an Olah). Even fixing a Korban is permitted only if it was already slaughtered. If Shtei ha'Lechem were lost, one may not slaughter the lambs Lo Lishmah. If a Todah was mixed with a Temurah, one may not transgress even Chulin b'Azarah in order to fix it (Menachos 80a). However, if Dam ha'Nefesh was mixed with Pasul blood, we do not put it on the Mizbe'ach to merit with the Kosher blood (Zevachim 78a)! That is a decree, like it says there. If a Kometz was mixed with the Shirayim, we do not sin (burn) the Shirayim to fix the Kometz (Menachos 23a), because they have different names. (It is like sinning for the sake of a different matter.)

Tosfos (48a DH Chet (2)): We should throw the blood in order to eat them raw on Shabbos! It seems that this is not a merit, so we do not sin for it. This is unlike the Heter to remove bread from an oven before he is liable for Chilul Shabbos (Shabbos 4a).

Sefas Emes (48a DH bi'Gemara Chet and Tosfos DH Chet): Eating the meat (even if it is on Shabbos) is not related to Shabbos, therefore, one may not transgress Shabbos for this! Also, once Zerikah was done, the Korban is Kosher, so one must wait until night to be Maktir the Chelev, so one may not eat the meat on Shabbos! Surely, we do not permit Haktaras Chelev during the day, or eating before Haktarah, in order to permit Zerikah on Shabbos!

Sefas Emes (48a DH b'Tosfos): Lo Lishmah is not an actual Lav. How does the Gemara compare Lo Lishmah to real Isurim? Even Chachamim hold that also Haktarah of Shirayim (l'Shem Etzim) is not a real Lav. It is a mere Isur. This is no source to transgress a real Lav in order to gain!

Sefer Chasidim (167): If one had relations with a Nochri (or Ervah), he should not try to fulfill Rav Yehudah's teaching (that full Teshuvah is when he is tempted with the same woman...) Perhaps she will desire to be with him, and he will sin! Chachamim did not say that one should bring himself to sin! Esther sinned in order to save Yisrael, therefore, she told Yisrael to fast three days and nights for her. Therefore, one who sinned with a woman should fast three days and three nights.

Poskim

Shulchan Aruch (EH 23:1): One may not emit seed needlessly. This is more severe than all Isurim in the Torah.

Chelkas Mechokek (1, citing Sefer Chasidim 176): If one fears lest he stumble with a married woman, his wife when she is Nidah, or any Ervah, Chas v'Shalom, it is better to spill his seed than to stumble with a woman. He needs atonement. He should sit in the ice in winter, or fast 40 days in summer.

Question (Chuikei Derech): In Siman 992, Sefer Chasidim says that if one's Yetzer Ra overpowers him with sexual desire, he should put his feet in cold water!

Beis Shmuel (1): The Zohar says that spilling seed is worse than all Aveiros in the Torah. According to Sefer Chasidim, this is not precise (there are worse Aveiros).

Rebuttal (Pesach Einayim Nidah 13a DH Amar): The Zohar says that wasting seed is the Aveirah that completed the Shi'ur of sin of Dor ha'Mabul and caused the decree of the flood. Sefer Chasidim discusses one who already has an erection and prepared to sin, and surely the semen will come out. He rules that it is better to separate from the Aveirah, and automatically the seed will be wasted b'Ones. This is like Yosef. When he separated from Potifar's wife, 10 drops of semen left him against his will (through his fingers - Rashi Bereishis 49:24, based on Tana d'Vei Eliyahu Pirkei ha'Yeridos 1), like Chazal say. If one is not in such a situation, deciding to waste seed is the worst Aveirah. This also answers Chuikei Derech's question. In Siman 176, Sefer Chasidim discusses one who has no time to bring cold water.

Meshiv Davar (2:44): In some places Chachamim permitted in order to help people repent. We accept Korbanos from sinners, to encourage them to repent (Chulin 5a), even though it says "Zevach Resha'im To'evah." We accept a Chatas for an Aveirah that he does not transgress b'Mezid, but not if he is a Mumar to that Aveirah, even if at the time he transgressed b'Shogeg. The Chazakah is that he will not repent from it. In Menachos we hold that one should sin in order to gain in the same matter. One may not transgress with Chatas for the sake of an Olah, but for the sake of a Chatas it is permitted.

Meshiv Davar (ibid.): If one sees that his Yetzer Ra is overpowering him, he should go to a place where he is not known and do what he desires (Chagigah 9a). Rashi and Tosfos explain that he should sin in private, and not come to Chilul Hash-m. R. Chananel, and Rashi in Mo'ed Katan in the name of Rav Hai Gaon, explain that surely he may not sin. Going away will help him to avoid sinning! They hold that we do not permit a small Aveirah to permit a big Aveirah. Beis Shmuel is like Rashi and Tosfos. However, not all light Aveiros are the same. Perhaps we do not permit a small Isur to avoid a great Isur if there is concern lest others stumble. One must consider well before permitting an Isur due to an Isur. Sometimes a doctor decides to cut off an aching hand lest it endanger the body, and sometimes he decides that it is better to suffer the pain and not lose the hand.