Is it permissible or forbidden to take the difference between the paid photography price and the actual photography price without the knowledge of the official in charge, and how can one atone for it if it is forbidden?

The questioner, in the mentioned transaction, is an agent for his company, and all the rights of the contract revert to the principal. Even if the buyer gifts him something, it is a right of the principal and is considered a reduction in price. He must repent to Allah and seek His forgiveness, and return this remainder to the company, for the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it has taken until it returns it."