He was entrusted with purchasing goods and bought them for half the value; is he permitted to take the remainder?

Question:
He says: Honorable Shaykh, may Allah grant you success. There is merchandise in the market worth one thousand riyals. When my brother goes to buy it, the merchant gives it to him for the same amount, one thousand riyals. But when I go, the merchant reduces the price by five hundred riyals. Is it permissible for me to take these five hundred riyals for myself?

Answer:
If he gave it to you as a gift, or even if he gave it to you for free, that belongs to him and he may do with it what he wishes. However, if he gave you a gift in the sense that he reduced part of its value specifically for you, there is no harm in that, because it is his property and he may dispose of it as he sees fit. But if he deceives people by selling to some at one price and to others at another, that is not permissible. He is required to sell the item at the same price at which it is sold in the market to everyone, to you and to others.