In my state, a comparable job is based on duties, responsibility, location, pay (and I would include benefits, since they are part of compensation)

If I accept a job with same duties, responsibility, location and pay, but no benefits, it's hardly comparable, since bennies can be between 15-20k a year for me. That's a big pay cut.

If it's not a position that you applied for and you've not gone through the hiring process, then you should not have to accept it. However, I assume that you submitted a resume and the agency looked at it and thought you'd be a better fit for that job. It might be worth investigating.