As subcontractor, we agree to install OSB on a section of ceiling instead of drywall as previously discussed. We agree to rush timeline in exchange for not being credited off change of scope (but no written agreement detailing this). Our contract calls for $x drywall work and $y OSB sheathing, but no specifications as to scope except our estimate. We are in process of collecting and now GC wants us to sign (-$500) Change Order agreeing to credit for change before they bill client (even with prior verbal agreement). If I don't sign, they won't bill and payment terms remain open since they haven't been paid yet. Should I refuse to sign CCO and file lien to expedite payment?