Each political subdivision required to make payments under § 88-221, may, in consideration of the employee’s retention in, or entry upon, employment after June 10, 1953, impose upon each of its employees, as to services which are covered by an approved plan, a contribution with respect to the employee’s wages (as defined in § 88-211 ), not exceeding the amount of the employee tax which would be imposed by the Federal Insurance Contributions Act if such services constituted employment within the meaning of that Act, and deduct the amount of such contribution from the employee’s wages as and when paid. Contributions so collected shall be paid into the fund in partial discharge of the liability of the political subdivision or instrumentality under § 88-221. Failure to deduct such contribution shall not relieve the employee or employer of liability therefor.