What is the ruling of Islamic law on one who attempts to reclaim mandatory income tax money imposed by a European state by deceiving it, knowing that this obligatory law is based on mutual consent and prior signing of a contract?

Some taxes are legitimate and must be adhered to, and it is forbidden to circumvent them. Others are unjust, and it is permissible to circumvent and evade them. There is no sin in evading illegitimate taxes, whether they are from a disbelieving or a Muslim state, because they are unjust, and it is permissible to remove injustice from oneself through circumvention. As for legitimate taxes, it is not permissible to circumvent them.