Is it permissible to study media in the office to earn additional income without the employer's knowledge?

The fundamental principle regarding work is that it is a trust (amanah), and Allah loves for a worker to perfect his work. Part of perfecting work is observing the conditions of employment. Therefore, one should not undertake work other than what they are assigned without the employer's permission, otherwise, it would constitute betrayal. Allah Almighty says: (O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]). Jurists have explicitly stated the impermissibility of a private employee working for anyone other than his employer, due to the disruption this causes to his work. A Muslim must seek lawful earnings, and such earnings are not permissible for him except with the employer's consent.