we provided materials for a project in Philadelphia, and the subcontractor made a warranty claim against it. the materials shipped over 6 months ago but to date we have not been paid. We agreed to provide labor and materials, at no cost to the sub, to correct the problem. Does the action of providing labor and materials at no cost extend our lien rights? And based on that, what would constitute proof (ie. a zero dollar invoice?) that we still have lien rights if they are challenged later?