The Secretary of State is authorized, subject to the conditions provided in this section and section 277d-9 of this title, to enter into an agreement with the appropriate official or officials of the United Mexican States for the construction, operation, and maintenance by the International Boundary and Water Commission, United States and Mexico, of a sanitation project for the cities of Calexico, California, and Mexicali, Lower California, Mexico, which agreement shall contain such provisions relating to a division between the two Governments of the cost of such construction and operation and maintenance, or of the work involved therein, as may be recommended by the said Commission and approved by the Government of Mexico and by the Secretary of State on behalf of the Government of the United States: Provided, That no such agreement shall be entered into until the governing body of the city of Calexico, California, has given assurances satisfactory to the Secretary of State that, so long as such agreement remains in force, the city of Calexico will contribute an equitable proportion as determined by the United States Section of said Commission, subject to the approval of the Secretary of State, of the costs of such construction, operation, and maintenance allocated to the United States.