Is it permissible for an employee who purchased supplies for his company to take a percentage of the money from the seller, or to agree with him on a higher price and submit a blank invoice to be filled with the new price?

If an employee is tasked with purchasing materials for their entity, they act as an agent for it and are entrusted with its interests. Their deception of their principal by informing them of a higher price than what they actually paid is a betrayal of trust. Allah the Almighty said: "Indeed, Allah commands you to render trusts to whom they are due," and the Prophet, peace and blessings be upon him, said: "Whoever cheats is not of us." Money earned in this manner is not permissible. For future permissible earnings, the employee can request a fee for their agency or purchase the materials with their own money and sell them to their principal at a profit. They must repent and return what they took or seek permission to keep it. Taking a percentage from the seller in exchange for choosing them is a bribe and is impermissible.