What is the ruling on raising the price of an item (a computer or one of its parts) when purchasing it for another person or for an institute, without informing them of the real price, and considering the difference as a commission or compensation for the effort of assembling the device and installing programs? And what must be done regarding past instances if that was unlawful?

You are considered an agent for the business owners, and your raising of the commodity's price excessively is a betrayal of trust and an unlawful consumption of wealth. You must repent from this by returning the differences to their owners or seeking their forgiveness, based on the Prophet's (peace be upon him) saying: "On the hand is what it has taken until it renders it," and "Whoever has wronged his brother concerning his honor or anything else, let him seek his forgiveness today before there is no dinar or dirham."