Is it permissible to fulfill a customer's request to reduce the value of goods on the invoice to evade taxes, and is this considered cheating?

If taxes are oppressive and unjust, it is permissible to cooperate in their removal and to evade them. This is not considered cheating, but rather an act of supporting the oppressed. It is also permissible to lie to save inviolable property from an oppressor. However, if taxes are permissible under Islamic law, it is not permissible to abstain from them or to use fraudulent means to evade them.