What is the ruling on selling by agreeing with the buyer on a specific price, and writing a lower price in the contract to evade taxes?

The agreement between the seller and the buyer to conceal part of the price to evade taxes depends on the ruling concerning taxes themselves. If the state imposes them rightfully to meet the needs of citizens and its resources are insufficient, then it is not permissible to evade them. However, if the state takes them unjustly or spends them improperly, then it is permissible to evade them to remove injustice and avert harm.