No mortgage shall be purchased by the Association in its operations under this section except pursuant to and in accordance with the terms of a contract or commitment to purchase the same made prior to the cutoff date provided for in section
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(d) of this title, which contract or commitment became a part of the aforesaid separate accountability, and the total amount of mortgages and commitments held by the Association under this section shall not, in any event, exceed $3,350,000,000: Provided, That such maximum amount shall be progressively reduced by the amount of cash realizations on account of principal of mortgages held under the aforesaid separate accountability and by cancellation of any commitments to purchase mortgages thereunder, as reflected by the books of the Association, with the objective that the entire aforesaid maximum amount shall be eliminated with the orderly liquidation of all mortgages held under the aforesaid separate accountability: And provided further, That nothing in this subsection shall preclude the Association from granting such usual and customary increases in the amounts of outstanding commitments (resulting from increased costs or otherwise) as have theretofore been covered by like increases in commitments granted by the agencies of the Federal Government insuring or guaranteeing the mortgages. There shall be excluded from the total amounts set forth in this subsection the amounts of any mortgages which, subsequent to May 31, 1954, are transferred by law to the Association and held under the aforesaid separate accountability.