The Secretary of Agriculture is hereafter authorized, in connection with the administration of the national forests, to enter into contracts for the procurement of services, materials, and supplies for the ensuing fiscal year, prior to the passage of an appropriation therefor: Provided, That such contracts shall aliquot the cost for such service by fiscal years and shall not be binding on the United States as to that part for the ensuing year unless and until an appropriation applicable to the payment thereof is made: And provided further, That all such contracts shall by their terms provide that the obligation of the United States is contingent upon the passage of an applicable appropriation and that no payment thereunder will be made until such appropriation becomes available for expenditure.