Employer Denies paying Overtime Hours already worked at End Client Location.

Hi there,

My employer contracts with other staffing companies, to place his employees at work. My situation:
End Client (where i worked as IT sub-contractor) --> Prime Agent Company --> Secondary Agent Company --> Employer.

Now issue is (Prime Agent Company - Project Manager to whom i was reporting to work at End Client location) made me work Over Time (without taking consent from End Client and his/her own boss/manager.

My employer sent invoice to Pay Overtime hours, to Secondary Agent and they refused..saying as it was NOT part of Approved Timesheet from Project Manager of Prime Agent Company.

I have email proof, when Prime Agent Project Manager asked me to send my Overtime Hours Worked at End Client Location.

As my employer did'nt got paid for my Overtime Hours worked at End Client Location, he tells me that he cannot pay me.

I need help here, as this has been going on over an year now. I spent many hours to resolve this issue with Prime Agent, Secondary Agent and My employer. And am worried as i am on H1B visa, they are forcing me to not go ahead in this Overtime payment claim, or else...I will jeopardize my Immigration Status in US.