Levelset filled a lien for us & now we've received notice from a attorney that the lien is invalid & is demanding...

It sounds like a common scare tactic that some people will use to try to worm their way out of paying you, Might be A good idea to enlist the services of A licensed and bonded collections firm to help you secure payment and ensure that lien is correct. Give me a call at 972-872-8783 I'd love to talk to you more on this and maybe help you get a resolution 

Hire an attorney and then we can go from there.