The head of an executive agency shall require that a contractor covered under paragraph (1) incorporate the requirement under such subsection into each subcontract for the procurement of property or services valued in excess of $500,000 at any tier under the contract. (b) Exceptions (1) Contracts for commercial items and commercially available off-the-shelf items The requirements under subsection (a) do not apply to contracts or subcontracts for the acquisition of commercial items or commercially available off-the-shelf items (as those terms are defined in sections 103(1) and 104, respectively, of title 41, United States Code). (2) Employees and independent contractors not covered The requirements under subsection (a) do not apply with respect to an employee or independent contractor who— (A) is covered by a collective bargaining agreement negotiated between the contractor or subcontractor and a labor organization representing the employee or independent contractor