All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
An employee is an individual who works for a certain party. Therefore, he/she is prohibited from taking up additional work during official working hours. Moreover, an employee is a trusted person who receives a wage in return for performing a certain task or dedicating their time to that. It is prohibited that he receives a wage for doing nothing, and he isn`t allowed to work for his personal benefit at the expense of his principal employer, even if he had his own clients, since he originally receives that wage against dedicating his time for that employer, and any money that he makes from his own clients or those of the company is the right of the company`s owner since Almighty Allah said{What means}, " And do not eat up your property among yourselves for vanities"{Al-Baqarah, 188}. In fact, hadn`t he been an employee of that company, he wouldn`t have made that money in the first place.
In principle, it is permissible for an employee to take up additional work outside official working hours as long as it isn`t with the company`s clients to prevent harming the owner of that company. In this regard, Prophet Mohammad(PBUH)said, " There is no injury nor return of injury"{Ibn Majah}. However, if it was stipulated in the employment contract that the employ is prohibited from taking additional work then he is obliged to adhere to that. Almighty Allah said, " O ye who believe! fulfil (all) obligations."{Al-Mai`da,1}. In addition, Prophet Mohammad(PBUH) said, " Muslims are on (i.e. stick to) their conditions."{At-Tirmithi}.
Accordingly, if the employment contract prohibits taking additional work, in the form mentioned in the above question, then an employ should abide by that condition unless he takes permission from whoever has the right and authority to grant him that. Otherwise, he will be in state of misconduct and violation of contract terms. And Allah knows best.