In addition to the apportionment required by sections 3103–3109 of this title, the head of each executive department shall, on or before the beginning of each fiscal year, apportion to each office or bureau of his department the maximum amount to be expended therefor during the fiscal year out of the contingent fund or funds appropriated for the entire year for the department, and the amounts so apportioned shall not be increased except upon the written direction of the head of the department, in which there shall be fully expressed his reasons therefor.