We are a subcontractor on a Federal project. We are nearly done on a $450k contract. The GC has rejected our last pay app of $40k and state they only owe us $5k. They have now revised our SOV (which has been used in 15 previous billings) and removed 5% of all work and listed it as "punchlist". They claim the remaining balance of our pay app will be held and fill the "punchlist" line item they have now created. They have also rejected change orders that we have written approval from them more than 1 year ago. We have had numerous change orders approved but never added to the contract value. We have billed them for those and have been paid, but they never added the costs to our contract value. Now they have contacted our tier sub directly and offered to hire them to complete the project. Is any of this legal?