Can landlord or agent/property manger charge 6% of the annual rent of a new tenant as a penalty to the previous tenant?

I recently had to vacate a rental 2 months prior to the end of the lease. The property manager/real estate agent is attempting to withhold $1044 of my security deposit calling it a "Rental Commission" as a cost for having to find a new tenant and basing it on 6% of the annual rent that will be paid by the new tenant. The new tenant was found and moved in 10 days after my vacating the property and the rent is $150 per month higher than what I was paying. The property manager has provided no proof of any cost incurred to find the new occupant or how he is entitled to the commission he has assessed. Can he legally do this?