Gemara

27b (Mishnah): One may falsely vow ...to tax-collectors, to convince them that it (what they want to take) is Terumah, or belongs to the king;

(Gemara) Question: (Why may one lie?) Shmuel taught that Dina d'Malchusa Dina (the laws of the country are binding)!

Answer #1 (Rav Chinena citing Shmuel): The tax collector in the Mishnah takes as much as he wants.

Answer #2 (d'Vei R. Yanai): The tax collector appointed himself (the king diko authorize him).

Bava Kama 113a - Version #1 (Mishnah): One may not change coins with or take Tzedakah from a tax collector.

Question: Shmuel taught that Dina d'Malchusa Dina!

Answer #1 (Rav Chanina bar Kahana): The Mishnah discusses a tax collector who takes as much as he wants.

Answer #2 (Tana d'vei R. Yishmael): It discusses a self-appointed tax collector.

Version #2 (Beraisa): One may not (some texts - wear 10 layers of Shatnez, one on top of the other, to) evade the tax;

R. Shimon says, R. Akiva permits evading the tax.

The Gemara asks why R. Akiva is not concerned for Dina d'Malchusa, and gives the above answers.

Version #3 (Mishnah): One may falsely vow ... tax-collectors, to convince them that it (what they want to take) is Terumah, or belongs to the king;

The Gemara asks why we are not concerned for Dina d'Malchusa, and gives the above two answers.

Answer #3 (Rav Ashi): The Mishnah discusses a Nochri tax collector.

(Beraisa - R. Yishmael): If a Yisrael and a Nochri (extortionist - some texts delete this) came for judgment ... if neither Torah nor Nochri law favors the Yisrael, we seek a ruse to exempt the Yisrael;

R. Akiva says, we may not seek a ruse. This is not Kidush Hash-m (sanctification of His name).

Inference: If not for Kidush Hash-m, we could use a ruse!

Rishonim

The Rif and Rosh (Bava Kama 40a and 10:12) bring the Gemara, but put Rav Ashi's answer on the question from wearing Shatnez to evade the tax.

Rosh: Even though one may not steal from Nochrim, he may dodge a debt owed to them.

Rambam (Hilchos Gezeilah 5:11): A tax collector is considered a thief if he is a Nochri, or appointed himself, or was appointed by the king but he takes as much as he wants.

Magid Mishneh: The Rambam holds that a Nochri, even if he was appointed by the king, is like one without a limit, for we assume that he takes more than he is entitled to. He holds that Rav Ashi's answer applies even to the Mishnah. (One may not take change from a Nochri tax collector, for he is assumed to be a thief.) He does not argue with the other answers. The other Meforshim explain differently. (They say that Rav Ashi permits vowing to a Nochri tax collector because one may evade debts to a Nochri. We do not assume that he is a thief.)

Question (Lechem Mishneh): If so, why does Rav Ashi support himself from the Beraisa of dodging debts of Nochrim? There is no difference between a Yisrael and a Nochri. If he takes extra, he is a thief, and one may evade the tax! Also, all agree that Rav Ashi does not argue with the other answers! However, perhaps others explain that if a Yisrael takes more than he should, one must pay him what he is entitled to.

Ra'avad (brought in Rashba Bava Kama 113a DH b'Moches): Rav Ashi's answer applies to the last two questions, about Sha'atnez and vowing. It does not apply to the Mishnah about taking change from a tax collector.

Rebuttal (Rashba): If we discuss a (Nochri) tax collector appointed by the king who has a limit, why may one vow to him aloud and Mevatel it in his heart? Devarim sheb'Lev (unspoken intents) do not count! This is permitted only regarding an extortionist! The Gemara asked this in Nedarim, and did not give Rav Ashi's answer! Proper texts put Rav Ashi's answer on the question from wearing Shatnez to evade the tax.

Gra (CM 369 15): The text of all the Poskim says 'd'Tanya'. Rav Ashi's answer is based on the following Beraisa, which permits dodging debts to Nochrim. The Rambam's text in Bava Kama said 'Tanya'. Rav Ashi answers that one may evade the tax when a Nochri collects it, for Stam Nochrim are thieves (Sukah 30a.)

Poskim

Shulchan Aruch (YD 232:14): One may vow falsely to a tax collector who is a Nochri, or appointed himself, or was appointed by the king but he takes as much as he wants.

Rema (CM 369:6): Even if it is known that the Yisrael takes more than the law, one may not evade paying what the law requires. This is like stealing from a thief, which is forbidden.

Beis Yosef (DH u'Mah she'Omar v'Chen): Even if it is known that a tax collector takes more than the king's law, one may not evade paying what the law requires. Just because he steals, this does not entitle others to steal from him. If one steals from a thief, he tastes theft (Berachos 5b).

Rebuttal #1 (Mabit 1:261, cited in Tur ha'Shalem footnote 54): Normally, if one cannot force a thief to return what he stole, he may steal from him only as much as the thief stole. However, a tax collector is different. People do not owe him, just he is entitled to collect from them due to Dina d'Malchusa. If he takes extra, he forfeits Dina d'Malchusa, and he is a thief. Therefore, one may evade the tax to save his own money. One answer in Bava Kama said that one may evade the tax when the collector has no limit. The other answers were that he was not appointed by the king, or he is a Nochri. This implies that even if the king appointed a Yisrael, if he takes too much, one may evade the tax. Tosfos says that everything he collects is theft.

Rebuttal #2 (Ketzos ha'Choshen 1): The Tur (YD 232) permits vowing falsely to a tax collector who appointed himself or takes more than his quota. This connotes he has a quota, just he takes more. We cannot say that one may vow only to avoid paying extra, for the Tur teaches a new case. The Gemara mentioned only one who appointed himself or has no quota! He learns from Perush ha'Rosh (28a DH b'Moches), who says that 'all that he takes is theft.' The king does not approve, so it is as if he appointed himself. The same applies to one who takes extra. In Yoreh De'ah (232:14), the Shulchan Aruch writes like the Tur. Here he does not mention this (only the Rema does). It seems that the Shulchan Aruch holds that it is total theft.

SMA (19): If a tax collector takes too much, one may lie to him about how much he has in order that the tax collector will take the proper amount.

Taz (6): If the only way to avoid paying extra is to totally evade the tax, he may do so. One need not give extra to a corrupt tax collector. He caused his own loss! However, at another opportunity he should discreetly give more than required, to compensate for what he should have paid.