What is the ruling on an engineer taking a percentage of money from the owner of a tractor with whom he works, when he did not stipulate it to him?

The amount given by the tractor owner to your friend is considered a gift or a present, and it is permissible for your friend to accept it, as long as it does not lead to favoritism that harms the company. This is because the objective is to nullify every means by which a person who takes money could show favoritism to the one from whom the money was taken.