What is the ruling regarding an extra 50 riyals found after distributing salaries, its owner is unknown, with the possibility of it being private money mixed with work money?

If there is no proof of ownership for this amount, then all possibilities are open. You must consider and act upon your best judgment. If you predominantly believe it belongs to the workers' salaries, then you are not permitted to take it, and you must strive to deliver it to its owner. Or, if you predominantly believe it belongs to the employer, then return it to the entity that authorized you to disburse its workers' salaries.