Manager has not control over it because it's fixed rate.

Hiring company pays a fix rate to fill positions, staffing firm makes the hiring at negociated rate after candidates accepted by hiring manager. It's either you make more or staffing firm makes more, so research to get the best rate for yourself. Depends on the pool of candidates, staffing firm may make up to 40% of the rate. Very very profitable bus. I think mine is getting 25%.

read this: http://labmice.techtarget.com/articles/contractingsecrets.htm