What is the ruling of Sharia on taking a commission without the client's knowledge when selling computer devices and their accessories, knowing that the invoice is issued by the company at a value that includes the commission, and that the seller specifies the device's specifications, purchases it, and receives it on behalf of the client?

It is permissible for a person to buy devices for customers and then sell them to them at a profit, provided that he owns the device first, due to the Prophet's (peace and blessings be upon him) prohibition of selling what one does not own. The customer must know that you are a seller and not a donor, and it is not a condition for them to know the amount of your profit. However, if you volunteer to search for and purchase something for someone, you are not allowed to increase the price.

There are two other permissible scenarios: either you act as an agent for a fixed fee, or as an agent for a percentage of the device's price. In these two scenarios, you only take what you have agreed upon, and any discounts or gifts associated with the devices belong to the person who appointed you as their agent.