Is it permissible to take a sum of money in exchange for buying something for someone at a low price, and then selling it at a higher price without their knowledge?

If someone appoints you as an agent to purchase an item, you are not permitted to add anything to its price, because you are a trustee. Any profit or reduction reverts to the principal, unless the principal allows you to take a portion of it. You may request a fee for the agency, or inform him that you will purchase the item and then sell it to him. Even a gift given by the seller to the agent is considered the right of the principal. It is stated in Maṭālib Ūlī al-Nuha that a gift from the seller to the agent is like a reduction in price and is attached to the contract because it belongs to the principal.

The Permanent Committee was asked about an agent who purchased an item at a lower price than agreed upon. They replied that it is not permissible for the agent to take anything from the principal's money except with his permission. The Committee added that honesty is obligatory in transactions, and it is not permissible for the agent to take an increase over the price at which he purchased the item, nor for the seller to write an unreal price on the invoice, because that is cooperation in sin and consuming people's wealth unjustly.