What is the ruling on a partner buying a faulty device from the company for a certain amount, repairing it, and then selling it for a profit, knowing that he paid the price of the faulty device to the partners and the money was distributed among them?

If you purchase this device from your remaining partners, meaning their shares in it, there is no harm in that. And if you own the device, there is no harm in disposing of it in any permissible way, such as repairing it and selling it at any price agreed upon, based on the Almighty's saying: "O you who have believed, do not consume one another's property unjustly, but only [in lawful] trade by mutual consent." and the Prophet's (peace be upon him) saying: "Indeed, selling is by mutual consent." However, if you purchase the device yourself as an agent for your remaining partners, this is not valid except with their permission and at the fair market price, because an agent is obligated to act in the best interest of their principal.