Whenever the commissioner shall alter or relocate any portion of any class I or class II highway and finds that there is further occasion to use such portion for highway purposes, he may reassign such portion to another classification; and upon reassignment of any such portion to a class V highway he shall determine, and allot, what sum, if any, the state shall contribute toward its maintenance. He may, in the exercise of his discretion, supervise and direct the expenditure of such sum.