Whenever local interests shall offer to advance funds for the prosecution of a work of river and harbor improvement duly adopted and authorized by law the Secretary of the Army may, in his discretion, receive such funds and expend the same in the immediate prosecution of such work. The Secretary of the Army is authorized and directed to repay without interest, from appropriations which may be provided by Congress for river and harbor improvements, the moneys so contributed and expended: Provided, That no repayment of funds which may be contributed for the purpose of meeting any conditions of local cooperation imposed by Congress, nor under the authority of section
560 of this title shall be made.