What is the ruling on money earned from secret additional work—without the knowledge of the original employer—in the field of car importing? Is this work considered permissible (halal) or forbidden (haram)? What are the consequences of disposing of such earnings? Do the rulings of ijara (leasing/hiring) or sharaka (partnership) apply to the initial employment system? Does the ruling differ if the additional work is performed during official holidays?

It appears from the question that the questioner is an employee who falls under the ruling of a private contractor (ajir khass). It is not permissible for him to work another job during official working hours unless the work is done during a holiday or outside official working hours, in which case there is no harm in it. As for receiving wages as a percentage of what is earned from the work, there is a difference of opinion among scholars regarding its permissibility. The majority prohibited it due to the uncertainty of the wage, while some scholars, such as Ibn Sirin and Sheikh Al-Islam Ibn Taymiyyah, permitted it if the compensation was a common share of what is obtained from the work. The ruling differs according to the type of transaction: If it is a private contract (ijarah khassah), then it is not permissible to work another job during official working hours. However, if it is a joint contract (ijarah mushtaraka) or a commission-based contract (ja'alah), then the worker is permitted to work for whomever he wishes.