Is it permissible for an employee to receive the price difference when purchasing devices for the company or for clients, or to provide technical support for clients whose contracts have expired in exchange for a sum of money, and can he use these funds to cover personal or company expenses, especially if he does not receive his full due from the company?

You purchase goods for the company as an agent, and it is not permissible for you to increase the price for yourself without informing the buyer, unless this was stipulated and the company agreed. The excess amount of the price belongs to its owner, not to you, as in the Hadith of Urwa al-Bariqi.

Providing technical support to those who have not renewed their subscription to the company, outside official working hours, is permissible for a fee. However, during working hours, it is not permissible.

If you have money belonging to the company or to others, you must return it to its owner. It is not permissible to give it away as charity or dispose of it as long as you are able to deliver it. If the company permits using it to pay for the commercial register, then there is no harm.

You are entitled to nothing from the company except what was agreed upon in the contract regarding salary and rights, and the contract is the law between the contracting parties.