A public agency may not authorize or make any payments to a contractor under a contract which contains or should contain the provision required by section two of this article unless such contractor has fully complied with such provision. Prior to such payment, the public agency shall require sworn certificates of compliance from all contractors, subcontractors and suppliers whose work involved the supplying of aluminum, glass or steel products. Payments made by a public agency to any contractor who did not comply with this article may be recovered by such agency.