Is there any suspicion of usury or unlawfulness in purchasing a plot of land at a price higher than its original value and paying the difference to the intermediary who did not own the land, and is Zakat obligatory on this land after the passing of a full year, and what is its percentage?

The agent who bought land and sold it without the principal's permission must give the profit to his principal, for the profit belongs to the principal, not to the agent. As for the validity of your purchase of the land, the sale by an unauthorized person (selling someone else's property without their permission) is considered void by some scholars. However, the more authoritative opinion is that it is valid but contingent upon the original owner's ratification. Based on this, you must inform the true owner of the land; if he approves the sale, it is valid, but if he rejects it, the sale becomes void, and you must return the land and reclaim your money. Finally, if you bought the land to preserve wealth and not for trade, then you do not owe zakat on it.