Is it permissible for my father to take a monthly financial return from the workers who work for themselves in his establishment, based on an agreed-upon percentage, regardless of profit or loss?

If your father brings in workers to work for others in exchange for money he takes, this is not permissible; because sponsorship (kafalat) is one of the contracts of benevolence ( عقود الإرفاق) for which it is not permissible to take compensation.

However, if he brings them in to work for him for a wage, and then hires them out to others and takes the difference between the two wages, this is permissible with the workers' consent, and the difference goes to the one who first hired them.