Any grant to, or contract or agreement with, a State agency executed before January 2, 1971, under which Federal financial assistance is available to pay all or part of the cost of any program or project which will result in the displacement of any person on or after January 2, 1971, shall be amended to include the cost of providing payments and services under sections
4630 and
4655 of this title. If the head of a Federal agency determines that it is necessary for the expeditious completion of a program or project he may advance to the State agency the Federal share of the cost of any payments or assistance by such State agency pursuant to sections
4626,
4630,
4635, and
4655 of this title.