They can't withdraw from an offer once made. Equally, you should be entitled to pay parity with co-workers from the same agency.

If this is the agency's style of personnel management, then I would suggest that you search for a new agency, sort out another job, and resign. Vote with your feet.

If you feel that the place you are actually working at is a good place to work, why don't you approach the boss informally and lay out your cards. Sound him out to see if he'd be willing to take you on full time, and pay off the agency. It's pointless to piss them off, especially if they are happy with the standard of your work. Try it, you might be surprised. But don't stay with an agency who does that to you.