Is it permissible for an employee to keep the profit difference if it exceeds the percentage set for him by the employer in the sale of engineering devices?

Excess profits are the company's right, and it is not permissible for the agent to take anything from them; because the increase in price is the right of the principal. So, if the agent buys for less than the specified price, the surplus belongs to the principal, as in the hadith (definition) of Urwah ibn Al-Ja'd, that the Prophet, peace and blessings be upon him, sent him with a dinar to buy him a sacrificial animal. He bought him two, then sold one for a dinar, and brought the other to the Prophet, peace and blessings be upon him.