What is the ruling about someone who works a job, they pay him cash and he doesn't pay tax on it. His employer pays him under the table. It is not a registered job. The employer tells him if you do so and so I will pay you this much.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Imposing taxes on citizens without any right is one of the major sins, be it from a Muslim or a non-Muslim country. This is taking people's money without any right, and Allaah has severely warned us against this. Imaam Ahmad reported that 'Uqbah Ibn Naafi' narrated that the Prophet said: "The one who forcefully takes taxes from people will not enter Paradise." Therefore, if imposing taxes (by the government) is without right (Islamicly unjustified), there is no harm on you to avoid paying taxes, or some of the taxes whenever that is possible provided that you do not cause any harm to yourself by doing this.

However, if imposing taxes is a right, such as imposing taxes to provide essential services, e.g. education, health care, and other services then it is not permissible for you to avoid paying these taxes, as they are taken (by the government) to provide services for you. Moreover, when a Muslim enters a country with a pact, he is obliged to abide by the law of the land that does not contradict the Islamic Law, and he should not help anyone to transgress its law, as that is part of the contract. Allaah says (which means): {And fulfil every covenant. Verily, the covenant will be questioned about.}[Quran 17:34].

Allaah knows best.