Gemara

4b (Abaye): Wherever the Torah said not to do something, if one transgressed, he accomplished something. If not, he would not be punished!

(Rava): Iy Avid Lo Mehani (his Aveirah does not accomplish anything). He is punished for acting contrary to Torah!

Question (Beraisa): If a Yisrael (raped a woman and) divorced her, he remarries her, and he is not lashed.

He should be lashed for acting contrary to Torah!

Answer: There is different, for it says "Shalchah Kol Yamav".

Question (against Abaye): "Kol Chelev Yitzhar (v'Kol Chelev Sirosh)" commands tithing from each species onto itself;

(Mishnah): If one tithed from one species on a different species, it does not take effect.

Answer (Abaye and R. Ila'a): "Reishisam" (plural) teaches that each species must be tithed by itself.

Question: According to Rava, why must it say "Reishisam"?

Nedarim 20a (Beraisa - R. Yehudah): If one transgressed his Nezirus, we don't permit him until he has observed Nezirus for as long as he transgressed.

(R. Yosef): Since it says that we don't permit him, a Beis Din that did so acted improperly.

(Rav Acha bar Yakov): We excommunicate the Beis Din.

Gitin 90a (Mishnah - Beis Shamai): One may divorce his wife only if "he found in her Ervas Davar (immorality)";

Beis Hillel say, even if she burned the food, he may divorce her - "he found in her Ervas (or) Davar (a matter)";

Question (Rav Papa): If he found neither Ervah nor a Davar and divorced her, what is the law?

Answer (Rava): If one raped a Besulah "he may not divorce her his entire life." (If he divorced her,) he must remarry her. Here, it does not say that he must remarry her.

Rishonim

Mordechai (Bava Kama 195): Efrayim complained against Yoel in a Nochri court, and asked them to force him according to Torah law. Due to this, Yoel complained against Efrayim (to the Nochrim), and caused him a loss. Maharam agreed that Efrayim did improperly, and he should be stretched on the pillar (and lashed) for going to a Nochri court first, even though he requested Torah law. He must be lashed or pay what the Rabanan deem proper. However, Yoel must pay the entire loss he caused to Efrayim. Efrayim did not cause any loss to Yoel, even though he began the Aveirah. Had Yoel lost due to this, whoever caused the bigger loss would pay the difference to the other, like the law of two men who damaged each other, like the Rif wrote.

Question: A case occurred in which Shimon married Leah. Her father Reuven gave her a dowry. She had a daughter, then she and her daughter died. Reuven claimed back the dowry in Nochri law. He knows that in Torah law, Shimon inherits his wife, but everyone in that city follows Nochri law, and it is as if they stipulated to do so, just like one may collect a Kesuvah from Arabian camels (in a place where people rely on them like land). Also, the king said that the child's property passes to his mother's heirs, and Dina d'Malchusa Dina (the law of the kingdom is binding).

Answer (Rashba 6:254): Every monetary stipulation is valid. The Yerushalmi says so about those who write that if she dies without children, the dowry returns to her father. I add that wherever this is the custom, one who marries Stam, it is according to the custom. However, if they do so because this is Nochri law, it is forbidden. The Torah forbids mimicking the Nochrim. Judgment must be "Lifneihem" (in front of Yisrael judges), and not in front of Nochri courts, even if it is a monetary matter and both parties agree, even for a matter that they judge like Yisre'elim. There is no support from collecting from camels. Letter of the law, one can collect a Kesuvah from Metaltelim. However, R. Meir says that women are hesitant about this, for Kesuvos are collected after a long time. In Arabia, they sell land to invest in camels! Dina d'Malchusa Dina does not permit following Nochri law in cases like this. If so, a Bechor will not inherit double, and daughters will inherit with sons. Dina d'Malchusa Dina is when the king wants Nochri judges to judge in his land. One who borrows without witnesses is not believed to say that he paid. One who collects according to this is a thief. Do not say that it is as if he stipulated 'I will trust the lender to say that I did not pay.' In their law, if a seller can prove that the item used to his, he can take it from the buyer. No one can live with such laws! Rather, it is only matters that are established in the country (even before the current king), and that are for the benefit of the king, but not for a Yisrael's case against another Yisrael.

Maharik (187): If a document was made in a Nochri court, do we apply laws of document to it, e.g. the bearer of a document has the lower hand, in which Nochri laws are opposite to Torah law? It is a valid document, due to Dina d'Malchusa, but the Re'em says that this is only for land, like the Mordechai said. It benefits the king that land is acquired through documents from his courts. Surely Dina d'Malchusa does not affect who has the upper hand. Even if Dina d'Malchusa applies to Metaltelim, this is only for taxes and royal customs. Surely it does not affect laws between individuals. If not, Torah law would be Batel! The Ran says so. We are Machshir a document from Nochri courts only because they are concerned for their reputations (and do not lie). Some say that if one accepted to go in front of them for judgment, he revealed that he wanted to be judged in front of them. I disagree. Their documents are Kesherim, but we do not follow their judgments! If a tax collector does unlike the king said, this is not Dina d'Malchusa. All the more so, we do not follow his laws against Torah law! Rava said that we collect from Bnei Chorin with a Persian document. If we followed their law, it would collect from Meshubadim! If all documents must be made in Nochri court, this does not show that the parties wanted to be judgde according to Nochri law! In a Teshuvah, the Rosh says that if a document says 'you may claim from me in the law of Yisrael or Nochrim', even though the scribes often write this, he may not go to a Nochri court, even if they judge this matter like Torah law. The Tur said in his name that even if they are in front of us, and the borrower says 'you may take me to a Nochri court', we protest. If he does not listen, we excommunicate him. Perhaps they write 'or in Nochri law' in case one party is powerful, and Beis Din cannot force him. (Then, Beis Din authorizes him to go to Nochrim.)

Maharik (154): A case occurred in which a man had a claim against his son. He complained to the Nochrim. Surely he must leave the Nochri court and remove all the damage it caused to his son. What he seized through the Nochrim is meaningless, like the Mordechai wrote. Even if one is not Moser the Yisrael himself, one may not seize through Nochrim at all. This is even if he shows no honor to their laws, and merely uses them to seize. Here, the father was Moser his son to Nochri law. I excommunicate him if he will not release what he received from the Nochrim within one day and consent to pick Yisrael judges and judge in front of them.

Darchei Moshe (CM 26:3): In the Mordechai's case, they were Moser each other, therefore he ruled for them like Yisrael law. However, perhaps when only one was Moser (and the Nochrim obligated him), we do not hear his case to judge it (and he must pay like the Nochrim said).

Poskim

Rema (CM 26:1): If one went to a Nochri court and was liable according to their laws, and then he summonsed his opponent to try the case in front of Yisrael judges, some say that we do not try the case. Some say that we try the case, unless he caused a loss to his opponent in front of the Nochri judges. The first opinion is primary.

Beis Yosef (DH Yisrael): Maharik connotes that if he already went to a Nochri court, we do not judge him. However, the Mordechai and Teshuvas Maimoniyos (Nezikin 15) connote otherwise.

SMA (7): Also, if he already paid according to Nochri law, even if he would pay more according to Torah law, we do not judge him again.

Gra (4): Because he transgressed, we do not take his case, just like Beis Din may not permit a Nazir (who transgressed, until he observed Nezirus). A Beis Din that permitted him did improperly.

Nesivos ha'Mishpat (Bi'urim 1): If he asked Nochrim to collect damages for him, and they fined him, we do not judge him again. The SMA says that the same applies if they imposed a monetary payment.

Nesivos ha'Mishpat (Bi'urim 2): The Tumim gives two reasons favoring the first opinion. 1) It is as if he accepted a relative or Pasul judge to judge the case; he cannot retract after the final verdict. 2) This is a fine for going to Nochrim for judgment. Based on the first reason, if Ploni brought David to a Nochri court, and they obligated Ploni because David bribed the judges, we try the case again. Even if one accepted a relative or Pasul to judge, and they erred, we judge again. Based on the latter reason, we do not try it again. The Poskim did not distinguish. This connotes the latter reason is primary. However, if David admits that he owes him according to Torah law, surely he must pay. Even if it is a Safek, he must go to Yisrael judges, lest he keep Safek theft.

Pnei Yehoshua (Gitin 90a DH Gemara): Rava holds that Iy Avid Lo Mehani. Why does he validate divorce without a proper reason? Regarding a rapist, he needed a verse to teach why divorce takes effect! I answer that Rava said Iy Avid Lo Mehani only when there is a Lav.

Rebuttal (Noda bi'Yehudah 2 EH 129): Rava says Iy Avid Lo Mehani even if one transgressed an Aseh, for the Gemara asked from one who tithed on a different species, or from bad on good! I say oppositely. Abaye and Rava argue only about one who transgressed a Lav. If the Torah said to do something, and he did not do it properly, all agree that he did nothing! Rav Papa asked also according to Abaye. The Levush wanted to disqualify divorce shortly before death (so that she will not fall to Yibum), since he divorces due to love, and not due to hate. He erred. It is not a condition that one must divorce due to hate. The Torah merely discussed the typical case.

Maharit (1:69): If an Isur was done, and it cannot be undone, we do not say Iy Avid Lo Mehani. E.g. if one went to a Nochri court, the judgment itself was not the Isur, rather, being in front of Nochrim. This cannot be undone. Once, a Tzibur made enactments in a Nochri court unlike previous enactments. The Nochri court was already honored, and this cannot be undone. Therefore, Iy Avid Mehani. Even though it opposes the previous enactments, Tosfos (6a DH v'Hashta) says that Iy Avid Lo Mehani applies only to Isurei Torah, but not to Isurim that people made.

Chasam Sofer (YD 6): He explains that Tosfos holds that Iy Avid Lo Mehani is a Chidush. Rava saw extra verses, and inferred that normally, Iy Avid Lo Mehani. We limit the Chidush to what Hash-m commanded. We say like this regarding Ein Isur Chal Al Isur (Shevu'os 24a).