why did amir leave money under the matress before leaveing with wahid's family instead of giving it to him?

Amir heard one of Wahid's wives scolding him for not leaving any food for the children; Amir realized that the boys had been staring not at his watch, but at his food. Before he and Farid left the next morning, Amir tucked a wad of money under a mattress for them to find.