The Secretary of the Army is authorized to receive from private parties such funds as may be contributed by them to be expended in connection with funds appropriated by the United States for any authorized work of public improvement of rivers and harbors whenever such work and expenditure may be considered by the Chief of Engineers as advantageous to the interests of navigation: Provided, That when contributions heretofore or hereafter made by local interests for river and harbor improvements, in accordance with specific requirements or under general authority of Congress, are in excess of the actual cost of the work contemplated and properly chargeable to such contributions, such excess contributions may, with the approval of the Secretary of the Army, be returned to the proper representatives of the contributing interests, unless the provision of law under which the contribution is made requires that the entire contribution be retained by the United States.