I'm a Realtor. Box 20 on my T4A contains the gross commission total. Where do I deduct the brokerage share that was never paid to me?

If you have income on a T4A in Box 20, you'll need to complete the T2125 - Statement of Business Activities that the T4A income has transferred to. 

Enter the T4A first, then go the Self-Employment section in TurboTax and choose to "edit" the business statement where this commission income has transferred to.  In the expenses section of this statement, you can enter these amounts as "Management or Admistration Fees" or under "Other Expenses" and specify what they are for.