to services performed before such effective date for which there were timely paid in good faith (and not subsequently refunded) to the Secretary of the Treasury amounts equivalent to the sum of the taxes which would have been imposed by sections 3101 and 3111 of the Internal Revenue Code of 1986 had such services constituted employment for purposes of chapter 21 of such Code. No agreement under this section may require payment to be made after the effective date specified in such agreement of any taxes with respect to services performed before such effective date. (d) Duration of agreement No agreement under this section may be terminated on or after the effective date of the agreement. . (b) Conforming amendments (1) Social Security Act Section 210(a) of the Social Security Act ( 42 U.S.C. 410(a) ) is amended— (A) in paragraph (20), by striking or at the end