Does an employee deserve a commission on his sales if there was no written agreement on a commission system and he left work before such a system was put in place, and how is this calculated Islamically, and what is the Shar'i (Islamic legal) position if the company later introduces a system that deprives him of this commission?

What determines the entitlement to commission is the contract between the employee and the company. If the contract stipulates the commission, then it is due; otherwise, it is not, unless there is a prevailing custom that necessitates its entitlement.

If the commission is due as a percentage of sales, this is a matter of scholarly disagreement.

In all cases, the employee is entitled either to the prevailing wage (ajrat al-mithl) or the agreed-upon percentage.

If the company issues a regulation prohibiting commission, this does not nullify the right of the employee who was entitled to it prior to the regulation, because it is a right that became a liability on the company. This regulation will apply in the future if both parties agree to it.