How valid is the threat of lien and seize and sell threat by an aircraft mechanic for a false claim of purported bills?

Can an aircraft repair shop in AZ register a lien against the aircraft based on simply the purported bills inconsistent with terms of a written contract; especially after the repair shop provided a written notice of the breach of contract for delays in completion and refusing to complete the remaining work unless the purported said amount is first paid? The right amount for the jobs done to date was already prepaid. How serious or lawful is their threat to seize and sell the aircraft to recover their purported and unearned bills inconsistent to the quoted amounts?