If you are entrusted to buy something and find it at a lower price, do you return the remainder?

Question : Please. Peace be upon you. She says: Honorable Shaykh, regarding the case where a woman gives someone money to buy goods for her, and the shop owner reduces the price of the goods with that money, she asks: If I kept it, is there anything required of me, Honorable Shaykh?

Answer : She has no right to it. So if the woman gave her one hundred riyals and said, “Buy me such-and-such item,” and then the shop owner reduced the price and sold it to her for eighty, it is obligatory for her to return twenty riyals to the woman.