To what extent is the wage permissible for an engineer who works on his private project, if he uses his company's tools and resources and spends his working hours on his private work without the knowledge of the company owner?

The employee is required to be present and leave at the agreed-upon time. It is not permissible for him to engage in private work during official working hours, or to be absent from work except with permission, or to use work tools for his private affairs. He is obligated to seek forgiveness from the company owner or compensate him for what he missed. If you perform a permissible task for this employee, you are entitled to the wage, and there is no sin upon you in using his work tools if you do not know their source, or if you know and he informed you that his manager authorized it.