If the sums appropriated under section
10409 of this title for any fiscal year and available for grants to States authorized under section
10402
(a) of this title are not sufficient to pay in full the total amounts which all States are entitled to receive under such section for such fiscal year, then the maximum amounts which all States are entitled to receive under such section for such fiscal year shall be ratably reduced. In the event that additional funds become available for making such grants for any fiscal year during which the preceding sentence is applicable, such reduced amounts shall be increased on the same basis as they were reduced.