Is it permissible to fast at work if it does not affect productivity?

There is no consideration for the employer's permission for an employee to fast voluntarily as long as the fasting does not weaken him from work. Scholars have stipulated the permissibility for a hired worker to fast voluntarily without the employer's permission if the fasting does not harm him in his work.

A slave is not prevented from voluntary fasting as long as it does not harm him in his work; how then can a hired worker be prevented?