When the Secretary
of the Interior shall find that any project approved by him has been completed or, if involving research relating to wildlife, is being conducted, in compliance with said plans and specifications, he shall cause to be paid to the proper authority of said State the amount set aside for said project. The Secretary
of the Interior may, in his discretion, from time to time, make payments on said project as the same progresses; but these payments, including previous payments, if any, shall not be more than the United States pro rata share of the project in conformity with said plans and specifications. If a State has elected to avail itself of the benefits of this chapter by preparing a comprehensive fish and wildlife plan as provided for under option (1) of subsection (a) of section 669e of this title
, and this plan has been approved by the Secretary of the Interior, then the Secretary may, in his discretion, and under such rules and regulations as he may prescribe, advance funds to the State for financing the United States pro rata share agreed upon between the State fish and game department and the Secretary.