There is authorized to be appropriated to the United States Section, International Boundary and Water Commission, United States and Mexico, such sums as may be necessary to defray such costs as may accrue to the United States arising out of any such agreement for the operation and maintenance of such project: Provided, That funds heretofore appropriated to the Department of State under the heading “International Boundary and Water Commission, United States and Mexico”, shall be available for expenditure for the purposes of this section and section 277d–6 of this title: Provided further, That any moneys received from the United Mexican States under the terms of any such agreement shall be available for expenditure in connection with any appropriations which may be available or which may be made for the purposes of said sections: And provided further, That moneys received from the city of Douglas, Arizona, pursuant to the provisions of said sections shall be available for expenditure in connection with any appropriations which may be available or which may be made available for the purposes of said sections.