What is the ruling of the Sharia on someone who undertakes private work online during long periods of free time at their job, and then donates the equivalent value of this time to their workplace, knowing that they have been issued a fatwa stating the impermissibility of private work during work hours?

Your time during working hours is owned by your employer, as you are a private employee. Therefore, it is not permissible for you to work for yourself or for another entity without permission, even if it is during your free time.

However, there is no harm in browsing the internet, reading books, or engaging in other personal activities if they do not affect your work, and you are not required to return a portion of your wages. It is preferable to seek permission from your employer.

If your personal work harms your employer, they have the right to demand compensation for the damage you caused.