Questions about salary negotiation

A staffing agency helped me get an interview, it is a contract-to-hire position. When the agency introduced me this opportunity, they told me the hourly rate and I agreed. But later I found the same openning online and the hourly rate is much higher (the same position posted by the same staffing company). Should I negotiate it with my stafffing agency before the interview or after the interview? Or I shouldn't negociate because I already agreed?
It seems after I agreed the rate they told me, then they sent my resume to hiring company and got me the interview.

Thanks a lot for your reply!