What is the ruling on paying a sum of money to a real estate office employee without the owner's knowledge to reduce the price of a plot of land?

The office employee is considered a special agent (Ajeer Khas) and a trustee (Ameen). What he did by facilitating the purchase process at a lower price is considered a breach of trust, and the amount paid to him is unlawful gain (Suht).

You must repent to Allah and inform the office owner of what happened. If he agrees, then so be it. If he refuses, then you must return the land and reclaim your money, or pay whatever the office owner deems appropriate.