If, during any one year, 75 per centum of the receipts are insufficient fully to meet the payments due the counties hereunder, the Secretary of the Treasury, upon certification by the Secretary of the Interior, may pay an additional sum from any surplus of 75 per centum of prior year receipts: Provided, however, That in no event shall the aggregate of payments during any ten-year period commencing with the period beginning July 1, 1940, exceed 75 per centum of the receipts deposited in the Treasury to the credit of the Coos Bay Wagon Road grant fund for such period: Provided further, That at the end of each ten-year period, any balance of the 75 per centum not required for payments to the counties shall be covered into the general fund of the Treasury of the United States.