Is it permissible for a Muslim to take the remainder of the money he unlawfully seized, if the head of the association responsible for this money forgives him?

You erred in taking money unjustly, and you did well in repenting, feeling remorse, and returning some of it. As for the remainder, you must verify whether the head of the association is authorized to waive it. If he is, then there is no blame on you. Otherwise, the waiver is not valid unless he personally undertakes to pay it, and you are obliged to pay the remainder, based on the Prophet's (peace and blessings be upon him) saying: "The hand is responsible for what it takes until it returns it."