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 did not 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 two answers.

Version #3: The Gemara asks, since Dina d'Malchusa Dina, why does a Mishnah permit vowing falsely to tax-collectors? It 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. In their text, Rav Ashi answered the question from wearing Shatnez to evade the tax.

Rif and Rosh: The Halachah follows Rav Ashi, for he is Basra.

Rashi on the Rif (DH Asur): The Tana'im argue about wearing Kil'ayim in order to look like a Nochri in a place where they do not collect from Nochrim.

Rebuttal (Rosh 11 and Hagahos Maimoniyos Yesodei ha'Torah 5:5): R. Akiva would not permit dressing to deny being a Yisrael, in order to evade the tax. One may not even wear shoelaces like the Nochri custom!

Defense (Nimukei Yosef): One must be Moser Nefesh for shoelaces only when the Nochrim seek to dodge the Yisrael custom.

Rosh (10,12): Because Dina d'Malchusa Dina, one may not steal from a Yisrael who bought collection rights from the king. All the more so, one may not swear falsely to him. If a Nochri collects, even though one may not steal from Nochrim, he may dodge a debt owed to them.

Rambam (Hilchos Gezeilah 5:11): If the king fixed a tax of a third or a quarter or limited amount, and appointed a Yisrael to collect it for him, and it is known that the collector is trustworthy and does not take extra, he is not called a thief, because the king's law is binding.

Question (Kesef Mishneh and Bedek ha'Bayis CM 369 DH v'Yesh): Why do the Rambam and Tur say that evading the tax from a Nochri is theft, even if the king is a Nochri? This is only dodging a debt! The Tur permits evading the tax when a Nochri bought collection rights from the king. The same applies when someone collects for a Nochri king!

Answer #1 (Kesef Mishneh): Perhaps here is different, for if the king finds out, in addition to Chilul Hash-m there is (mortal) danger. If a Nochri bought collection rights, the king does not lose, so there is no danger.

Rejection and Answer #2 (Mishneh l'Melech): The Rambam does not mention danger. Rather, when someone collects for the king, Dina d'Malchusa obligates paying, therefore one may not dodge the debt.

Rosh (3:11): Because Dina d'Malchusa Dina, one may not cheat to evade paying the tax to one who bought from the king the right to collect it. Dina d'Malchusa is Dina because the king owns the country. The same applies to a commoner who owns land; one may not benefit from it without his consent and allowance.

Poskim

Shulchan Aruch (CM 369:6): If the king authorized a Yisrael tax collector to take a fixed amount (Rema - even if he fixed a larger amount for a Yisrael than for a Nochri, this is called fixed), and he is known to be trustworthy and does not take extra, he is not called a thief, because the king's law is binding.

Source (Maharik 194, cited in Beis Yosef DH Kosav Maharik): Since all Yisraelim pay the same amount, this is Dina d'Malchusa.

Gra (17): Rashi on the Rif said that R. Akiva permits wearing Kil'ayim in order to look like a Nochri to evade the tax. The Rosh rejected this because one may not pretend to be a Nochri. We infer that Dina d'Malchusa applies to a tax that is only against Yisraelim!

Shulchan Aruch (ibid.): Further, one who evades the tax transgresses (Lo Sigzol), for he steals the king's portion, whether the king is a Yisrael or Nochri.

Beis Yosef (DH u'Mah she'Omar Bein): If we distinguish between a Yisrael or Nochri king, the Gemara should have used this to answer why one may evade the tax.

Gra (18,19,23): If we would say that one may evade paying one who collects for the king, one could evade also a Yisrael who bought collection rights, for he has no more rights than the one he bought from! The Halachah does not follow this. We hold like the Ran and Rosh, who forbid only when a Yisrael bought collection rights.

Shulchan Aruch (ibid.): Likewise, if a Yisrael bought from the king the right to collect the tax, one who evades it steals from the Yisrael who bought it. If a Nochri bought the right to collect the tax, one may evade it, just like one may dodge a debt owed to him, when there is no Chilul Hash-m.

Beis Yosef (DH u'Mah she'Omar Aval): Rashi explains that when there is no Chilul Hash-m, one may dodge a debt to a Nochri, for this is not true theft. E.g. one may children of a deceased Nochri that he paid their father. The children will not know that he lies. The same applies to evading the tax.

Rema: Some say that even when a Yisrael collects, if he did not buy collection rights, merely he collects for a Nochri king, even though one may not evade the tax due to Dina d'Malchusa, the collector may not force one who evades it to give. Evading is like dodging a debt, which is permitted. However, if the collector fears (punishment from) the king, he may force him.

Source (Beis Yosef DH v'Chosav ha'Ran, citing the Ran): A Yisrael collector cannot force another Yisrael, unless he fears punishment. Dina d'Malchusa Dina does not make it stronger than a debt, and we hold that one may dodge a debt! (The Ran brought in Chesronos ha'Shas (28a DH b'Moches) says so. Perhaps our version was censored - Hagahos Tur ha'Shalem 59.)

Erech Lechem: Since one may dodge paying, one may not tell the king about property of someone who owes to him. One who tells is a Moser (informer).