What is the ruling on working "under the table" (without paying taxes to the state) in Denmark, while receiving a salary from the state?

It is not permissible to reside in countries where evil acts are widespread, except out of necessity or an urgent need, and with assurance of being safe from falling into what is unlawful.

Regarding the payment of taxes: If the employer pays them in exchange for a reduction in wages, the matter is clear. Otherwise, if they are of the type permitted to be taken or if you agreed to them through the residence contract, then you are obligated to fulfill them, based on the Almighty's saying: "And fulfill [every] covenant. Indeed, every covenant will be questioned" and "O you who have believed, fulfill [all] contracts." The obligation to pay becomes even more emphasized if not paying leads to humiliation or degradation, or gives a bad impression of Islam, based on the Prophet's (peace and blessings be upon him) saying: "It is not befitting for a believer to humiliate himself."