Is it considered usury for my father to give me 200 riyals in exchange for 150 riyals I spent buying household necessities with the intention of charity?

Filial piety is one of the greatest causes for entering Paradise, and among the acts of piety is spending on one's parents. If you buy items for your father for a certain amount, and he gives you a larger amount, there is no harm in that. He is donating the excess to you. There is no usury (riba) in this matter; rather, both of you are rewarded. You intended to donate to him, and he donated the excess amount to you.