Is it permissible to sell the device for 1000 pounds and inform the partner, who no longer contributes with his time or effort, that it was sold for 800 pounds?

It is not permissible to lie, defraud a partner, betray a trust, or consume wealth unjustly, for Allah Almighty says: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] trade by mutual consent," and His saying: "Do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." And for the saying of the Messenger of Allah, peace and blessings be upon him: "Indeed, your blood, your wealth, and your honor are sacred to you," and "Whoever cheats is not one of us," and "Deceit and treachery are in the Fire." The lack of a partner's participation should be addressed by obliging him to fulfill his duties, or by amending the contract by mutual agreement, or by dissolving the partnership.