Paid contractor invoice. Later Amex dispute found in my favor and contractor refunded then placed lien for non payment

One invoice was for poor work which the contractor promised my wife and I would be refunded because he agreed the work was poor and not necessary ( because he sent a young trainee to our urgent response). The other invoice was a bit earlier which I was asking questions regarding the need for the work carried out and the Freon added (unnecessarily). I could not get email responses so I went to Amex - to get his attention. I assumed no issues with first invoice and just wanted a response to my questions for other invoice . I had 2 independent HVAC contractors review the work and state the Freon and work was unnecessary and probably not done (or not done properly). My emails to contractor got one response. He said “he does not do emails) and hence will not respond. This is when I went to Amex. Amex found in my favor and told me that the contractor voluntarily agreed with the finds of Amex and refunded my charges. Contractor then , a week later, issued notice of lien on my house if not paid in full . This included the invoice he promised my wife and I that he would refund. What are my options. He has obviously done this before because Amex has confirmed that a few contractors now do this to take Amex out of the loop and go straight to the home owner. My point is that I did pay. Amex then disputed charge directly with contractor and contractor refunded. This should be done and dusted. Contractor did not have to refund to Amex if they were convinced that their charges were correct. They decided to do so. Their lien states that I have not pain for services carried out . But I did pay.