What is the ruling on an employee selling free tickets that come with a package of tickets designated for sale, in order to make personal profit, despite the supervisor's instructions to sell the entire package? Is this considered theft, embezzlement, or fraud? And what should he do with the money he earned, knowing that he regrets his action and does not currently possess the amount?

You must sincerely repent for taking the money by breaching trust, and return any excess beyond what is rightfully yours to its owner. As for what is rightfully yours from him, which is the wages for the days you worked with him, that is yours. If you do not have the money to return it, it remains a debt upon you until you repay it or are absolved from it. It is not permissible for you to give it away as charity as long as its owner is known.