Is it permissible to keep the extra money a worker earns by adding value to the price of merchandise without the merchant's knowledge?

What this worker is doing is treachery and wrongfully devouring wealth. Almighty Allah has forbidden treachery. The worker is an agent for the employer, and an agent is not permitted to take the increase for himself; rather, the increase belongs to the principal. He must return it to its owner along with repentance.