In Texas, on a Residential Construction Project, can a GC claim or file a Lien on non contractual change order amounts?

I am a property owner and the GC is just finishing up a residential construction project on our property after a fire. We have a signed contract between all parties for #X dollar amount. The contract, provided by the GC, clearly states that any and all amendments or additions to the original scope or contractual amount require a formally signed change order by all parties. No such change orders exist and were never agreed upon or signed. We are requesting a conditional final payment lien waiver for the remaining contractual amount, which we have no problem issuing to the GC. However, the GC is only granting a conditional progress payment waiver even though this payment would fulfill the entire contract value, claiming that they have spent over the original Contracted value. The additional amount spent was never formally agreed upon or signed via formal change orders as required by the contract. Does the GC have the legal right to claim or file a lien for this additional amount? How can we ensure we are not liable for this additional amount? This is a result of poor cost management and the GC not following their own contract and formal change order processes.