Someone appointed him to buy something for him; is it permissible for him to increase the price without that person’s knowledge?
Question

                Is it permissible to take a sum of money in return for doing a service for someone? For example, someone asked me to buy something for him, because I have knowledge of such things. Then one of my friends agreed to give me that thing for a cheap price, because of my standing with him. Can I increase the price a little for the one who asked me to buy it and keep the difference for myself without him knowing?.
Praise be to Allah.
    If someone has appointed you to buy something for him, you do 
    not have the right to increase its price, because you have been entrusted 
    with that, and the basic principle is that any loss or gain goes back to the 
    person who appointed you, unless he allows you to take some of it. 

    It is permissible for you to ask him for a fee in return for 
    acting on his behalf, or to tell him that you will buy the item and then 
    sell it to him. If the seller gives it to you for free, then according to 
    the scholars it belongs to the one who appointed the other to buy it, not to 
    the one who was appointed (the agent). 

    It says in Mataalib Uli’l-Nuha (3/132): “If a seller 
    gives someone who is acting as an agent a gift, after he bought something 
    from him, it is like decreasing the price, so it is part of the transaction 
    and so it belongs to the one who appointed him.” This means that any 
    reduction in price is for the benefit of the one who appointed the person to 
    buy the item; the same applies to any gift that is given by the seller to 
    the agent. 

    The Standing Committee was asked: 

    Someone else appointed me to buy something for him, and its 
    price was five pounds, for example, but the man gave it to him for four and 
    a half pounds. Does he (the agent) have the right to take the remaining 
    money, which is half a pound (fifty pence), or not? 

    The Committee replied: 
This is regarded as 
    appointing someone else to do something on one's behalf, and it is not 
    permissible for the person appointed as an agent to take anything from the 
    wealth of the one who appointed him, except with his permission, because of 
    the general meaning of the evidence which states that the wealth of a Muslim 
    man is haraam to others unless he gives it willingly. 

    Fataawa al-Lajnah al-Daa’imah, 
    14/273. 

    It also says (in Fataawa al-Lajnah al-Daa’imah, 
    14/275): 
The Muslim must be 
    honest in his dealings. It is not permissible for him to lie and to take 
    people’s wealth unlawfully. That includes the case of a man who appoints his 
    brother to buy something for him; it is not permissible for him (the agent) 
    to take anything more than the price that he paid for the goods. Similarly 
    it is not permissible for the one who sold it to him to write a price other 
    than the real price on the bill in order to deceive the one who appointed 
    him, so that he will pay more than the real price and the agent will pocket 
    the difference, because this is a kind of coopering in sin and iniquity, and 
    consuming people’s wealth unlawfully. A Muslim’s wealth is not permissible to others unless he gives it willingly. 

    And Allaah knows best.