Do I have to pay the taxes on the gross wages in my W2 if my agent takes a cut out of it and how do I account for this on my taxes?

I paid taxes on the gross wages on my W2.  My checks were sent to my agent who then took their cut and then sent me checks for the remainder.  Why do I have to pay taxes on their cut?  How do I account for this on my taxes?  I have both W2's and 1099's for my income as an dance performer.  I put the 1099's in the Business Income along with all my expenses. A tax professional told me I had to keep the deductions with the source of the money (W2 or 1099). I am confused.