I am a contractor for IT services.
Two of the companies I do work for charge a 10% fee, deducted from what I am paid. These companies (comp A) find work, offer it, I (me) bid on it, if the end customer (comp B) accepts the bid I (me) do the work and the comp A takes 10% from what comp B pays. Comp A pays me, comp B pays comp A.

Is that service fee deductible for a small business? If so, does it fall to the Professional Services deduction?

Thank you

To keep this simple let's suppose that Company B pays $100 to Company A, and then Company A pays you $90. Does Company A issue you a 1099 to document what they paid you? If so, the amount they show you received should be the $90, which you report to the IRS as revenue, in which case you do not deduct the $10 fee.