Is there sin upon an employer who requests an hour to perform Jumu'ah prayer but does not attend the entire sermon, and is his prayer accepted, knowing that this hour is not free of charge?

If the muezzin gives the call to prayer for Jumu'ah after the Imam has sat on the minbar, it becomes unlawful to occupy oneself with anything other than hastening to it. You are obliged to strive to attend Jumu'ah if you are able. If the company does not permit you to do so, then you are excused, and you must attend as much of Jumu'ah as you can. An employee is excused from missing Jumu'ah if the employer does not grant permission. You should seek employment that allows you to fulfill your religious obligations.