If the Federal Unemployment Tax Act [26 U.S.C. § 3301 et seq.] is amended by the Congress of the United States to permit a maximum of credit against such federal tax higher than the 90 percent maximum rate of credit now permitted under § 3302(c) of the Internal Revenue Code [26 U.S.C. § 3302(c)] to an employer with respect to this chapter, then for any such employer, liable under the federal statute, the employer's contribution rate under this chapter shall be that determined in § 3350(5) of this title plus the 3/1 0 of 1 percent additional offset credit permitted under federal law.