If you paid someone who was not a Realtor and was not a GST/HST Registrant, you should not have paid any GST/HST on this fee. You can claim this as an "Advertising & Promotion" expense on the T2125 - Statement of Business Activities. 

Fees paid to another Realtor can be entered as "Subcontracts".

Here's a very informative article from the TaxPerts Group which details Realtor Income & Expense: