Is it Islamically permissible to pay 20% of the apartment's value in cash without declaring it, knowing that the seller will only declare 80%, and that this method is the customary practice for sales in Morocco?

If tax evasion is intended as a stratagem to negate a legitimate right, then it is not permissible to assist the seller in concealing part of the price, due to the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." However, if taxes are forbidden, then it is permissible to use stratagem to evade them and to assist others in doing so.