Assalaamu alaykum. I have come to know that if we have finished our work and still have free time, it is permissible to do our personal activities, and we are entitled to the salary if we stayed available during the time prescribed. What if I use this spare time to support my freelance work? This means that I am getting paid for freelance work that I do during the regular job time. I have heard that it is impermissible to do personal work that is paid during work hours even if we stayed available in the office of the regular job. Is this true? Is the salary from both jobs halal?

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.

The effort of the hired worker during working hours belongs to his employer, and he is not entitled to spend his effort in working for himself or for others except with the permission of his employer.

Az-Zaylaʻi wrote, "The Ajeer Khaas (private worker) is called thus because he is hired by one employer as a private worker to carry out a specific task during a specific time interval, and it is impermissible for him to work for another employer, because his effort in that specific time is exclusively owned by his employer, and the salary is in return for that effort..." [Tabyeen Al-Haqaa'iq]

Kashshaaf Al-Qinaaʻ reads, "He – i.e. the private worker – is not entitled to work for another employer because it entails depriving the employer of what he deserves by virtue of the contract."

The fact that the employee finished his work duties and has free time does not make it permissible for him to do work for others or for his own personal benefit as long as the employer has not given him permission to do so; and what is permitted by `Urf (customary practice of people in your community) substitutes explicit permission. The employee does not own this time so that he spends it as he wills or earns money during it, because this time belongs to his employer. So, what his employer permits him to do during this time, he may do it, and what his employer does not permit him to do, he may not do it.

The Kuwaiti Encyclopedia of Fiqh reads, "...so if he is a private worker, he is obliged to make himself available to his employer and allow him to receive the work agreed upon between them in full in the working hours, and he refrains from working for another employer during these hours."

If the employee worked for another employer during the working hours without his employer's consent, scholars held different views regarding the legal effects incurred by his action.

Kashshaaf Al-Qinaaʻ reads:

"If the private worker worked for other than his employer and his employer was harmed by that (incurred loss), his employer has the right to demand of him to return the value of that loss that he incurred by his working for another. (Imaam) Ahmad was asked about a man who hired a private worker to carry firewood on two donkeys every day. The worker carried firewood on the two donkeys for his employer and on donkeys for another man and received his wages from him. (Imaam Ahmad gave his fatwa, saying that) If working for the second employer incurred loss or harm on the first employer, the worker must return part of the wages in proportion to the loss. Ibn Qudaamah said, 'The apparent meaning is that the employer should reclaim part of the wages in proportion to the harm incurred on him because the employee engaged in other work. It could mean that the employee should give the employer the money he earned from the other work.' Al-Qaadhi said, 'It means that the employee should give his employer the money he has earned through working for another employer, because the private worker's effort during working hours are exclusively owned by the employer.' It is deduced that if the employer is not harmed (does not incur loss) by the employee's engagement in other work, the employee is not obliged to return any part of the wages because the employer has hired him to carry out a specific task and he did it as required." 

Allah knows best.