Gemara

(Rava): Regarding Arel, Haza'ah and a knife, Chachamim upheld their enactments even regarding Kares (preventing bringing Pesach). Regarding Onen, Metzora and Beis ha'Pras, they did not uphold their enactments regarding Kares.

Avodah Zarah 13a (Beraisa): One may go to a fair of Nochrim and buy animals, slaves, houses, fields and vineyards;

He may go to their courts, for this is like saving from them.

Tosefta (Mo'ed Katan 1:12): We may buy from Nochrim fields, houses, vineyards, slaves and animals, for this is like saving from them. He writes and enters the courts.

Gitin 88b (Beraisa - R. Meir): "These are the judgments that you will put Lifneihem (in front of Yisrael judges)", but not in front of Nochri courts. One may not go for judgment in front of Nochrim, even if their law (in the case) is the same as Torah law;

Rishonim

Rambam (Hilchos Avodah Zarah 9:14): One may go to Nochri fairs and buy from them animals, Nochri slaves, houses, fields, vineyards. He writes and enters their courts, for this is like saving from them.

Kesef Mishneh: Rashi explains that one may bring documents into the Nochri courts to sign them, even though this honors them, and perhaps they will thank their idolatry. It is permitted, for this is like saving from them. This enables him to have witnesses in case there will be protests. 'This is like saving from them' refers only to bringing documents into the Nochri courts.

Hagahos Ashri (Mo'ed Katan 2:18 DH v'Afilu, citing Or Zaru'a): One may buy houses from a Nochri on Shabbos. He shows to the Nochri a wallet of coins, and the Nochri signs and brings the document to the court.

Poskim

Shulchan Aruch (OC 539:12): We may buy houses, stones, slaves or animals [during Chol ha'Mo'ed] only l'Tzorech ha'Mo'ed.

Bi'ur Halachah (DH Ein): The Tosefta concludes 'he writes and enters the Nochri courts.' It says like this in Avodah Zarah 13a regarding going to Nochri fairs. Rashi explains that this refers to going to their courts. It is permitted, for it is like saving from them.

Shulchan Aruch (YD 149:3): One may go to a fair of Nochrim and buy animals, slaves, houses, fields and vineyards. He may write and go to their courts.

Beis Yosef (DH ve'a'Ha): Rashi explains that one writes documents and takes them to the Nochri courts, the place of their great people and judges, for them to sign, even though this is honor and glory for them, and perhaps he will thank his idolatry. It is permitted because it is like saving from them, so he will have witnesses to save him from protests.

Prishah (17): The Tur explains "he is Matzil" to explain the Heter to buy. It means that he separates the Nochri from things that last. This is why one may buy slaves, but not garments or Kelim. If "he is Matzil" explains the Heter to go to Nochri courts, how does it explain the Isur of garments and Kelim?!

Bach (Kuntres Acharon DH u'Mah she'Chasav ha'Rav): R. Yonah says that the only Heter to go to Nochri courts is when there was a Mitzvah to buy, to diminish from Nochrim (e.g. land). This does not apply to buying garments and Kelim. This benefits and richens them! Even if the Yisrael benefits, since there is no Mitzvah, one may not go to Nochri courts for them. This is obvious. I needed to write it only because the Prishah explained totally wrong.

Shiyurei Berachah (2): R. Yeshayah ha'Rishon of Triani (Teshuvah 99) says "making a document in Nochri courts is not forbidden due to "Lifneihem", and not in front of Nochrim. This is not a Mishpat (monetary case). He merely needs to write a document! The only concern is for honoring them, and the Nochri will thank his idolatry. Since the Yisrael intends to save his money, Chachamim did not forbid. Rashi says so. And even if it were Mishpat, the Isur is only when his opponent is a Yisrael who would heed Beis Din, but not when his opponent is a Nochri. And even if it were forbidden, we may not compare Isurim mid'Rabanan to each other. Chachamim weighed which Isur is overridden, and when, like it says in Pesachim 92a and several places. We may not be Mevatel their words until we have a clear proof of the matter itself, without comparing one matter to another. They knew where to be stringent and where to be lenient."

Teshuvas Beis Yosef (Kesuvos 10): A case occurred in which a dowry document written by a Nochri public scribe said that if she dies, half the dowry will go to her son. If it is a clear custom in the area to write Kesuvos through Nochri courts with this Tanai, it is binding. The Rambam is Machshir all documents of Nochri courts, except for gift documents. Ba'al ha'Itur, the Ramban and Rashba are Machshir even these, like Shmuel, who says that Dina d'Malchusa Dina (the law of the kingdom is binding). The Magid Mishneh and Rivash (203, 493) say that all Acharonim rule like this. The Tur (CM 65) brings from the Rosh that if the king established that all documents must be made through the Nochri courts, even gift and admission documents are Kosher, due to Dina d'Malchusa. Chachmei Modina were Machshir a dowry document not in front of the judge. This was not due to the custom not to require this for documents that are merely for a proof. The Mordechai wrote that there are customs that we do not rely on. Only when the Halachah is shaky, we follow the custom. I disagree. Many Mishnayos say that the Halachah follows the custom! Only regarding a small number of customs we say that the Halachah does not follow them.

Maharshdam (CM 304): If the custom in the city is to write documents through a Nochri scribe and witnesses, this is like any monetary stipulation, which is valid. It must be a clear custom that all in the city know, and it was established by Chachamim. The Rosh says so, and R. Yerucham brings so from the Rashba. Maharik brings it often. If the custom is to be Machshir even outside the courts, we follow it.

Teshuvas Bo'i Chayei (CM 1:158): If the custom is to go in front of Nochri courts for judgment, what is the law? Does such a custom override the Halachah? Or, perhaps we do not follow such a custom. I say that even the one who says that a monetary custom overrides Halachah, even if there is no proof from the Torah, here we do not follow the custom, for it is an Isur to go to Nochri courts for judgment, due to 'Lifneihem, but not in front of Nochri courts.' Almost all Poskim say that a custom against an Isur does not help, even if it is a clear custom established by Chachamim. If it is Mevatel any Mitzvah of the Torah, all agree that we do not follow it. Mahari ben Lev, Maharashdam, the Radvaz, the Beis Yosef, Mahari ha'Levi, Mishpat Tzedek and Gidulei Terumah (362) say that we follow a clear custom to write documents through Nochrim if Chachamim established it. Mishpetei Shmuel argues and considers this a tinge of Bitul Din Torah. There is very different. It is as if they stipulated to accept documents of Nochri courts, just like both parties may accept a relative or Pasul to be a judge or witness. Who will protest if they accept this? Here (going to Nochri courts for judgment), it pertains to an Isur, even if they rule like Torah law. This is like stipulating to do an Isur. The Tur (CM 26) brings Teshuvas ha'Rosh, who says that one may make his money Hefker and give it to whom he wants. No one can pardon and allow him to do an Isur.

Bo'i Chayei (ibid.): The Rambam forbids even if both parties want to go to the Nochri courts. R. Yerucham wrote "if one litigant wants to go to the Nochri courts, we do not heed him." The Rif wrote in a Teshuvah that we excommunicate him. Why did the Beis Yosef bring the Rambam and R. Yerucham without saying that they argue? Perhaps he holds that also R. Yerucham forbids even when both want, just he teaches that when only one wants, we excommunicate him. The Rema clearly holds like this. Alternatively, perhaps the Isur when both want is only if the lender has no more rights in their law than in ours, but if he has more rights in their law, and both consent, it is permitted.