The term “employment” does not include:
(1) Service performed in the employ of the United States or any instrumentality of the United States exempt under the Constitution of the United States from the payments imposed by this law, except that to the extent that the Congress of the United States permits states to require any instrumentalities of the United States to make payments into an unemployment fund under a state unemployment compensation law, all of the provisions of this law are applicable to the instrumentalities and to service performed for the instrumentalities in the same manner, to the same extent, and on the same terms as to all other employers, employing units, individuals, and services: Provided, That if this state is not certified for any year by the Secretary of Labor under 26 U.S.C. 3404, subsection (c), the payments required of the instrumentalities with respect to the year shall be refunded by the commissioner from the fund in the same manner and within the same period as is provided in 21A-5-19 of this code with respect to payments erroneously collected;