Since the realtor is required to issue you a 1099-MISC to report the rental income paid to you, you'll have to wait and see if it's included in that 1099-MISC. If it is included, then you'll report it as rental income and then negate taxes on it by reporting the fee as a rental management expense.
If it is not included in the 1099-MISC, then you will not and can not report the realtor fee as an expense.
