Whenever any person herein called the contractor shall contract with another for the performance of any work which the contractor has undertaken to perform, the contractor shall be deemed an employer and shall become civilly liable to employees engaged in performance of work under such contract for the payment of wages (but not for the payment of liquidated damages) as required by Sections 2 and 4 of this act¹ whenever and to the extent that the employer of such employees fails to pay such wages; provided, however, that the employer of such employees shall be liable to the contractor for wages paid to such employees by the contractor under this section.