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

Question

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.