(a) The Secretary shall dispose of materials under this subchapter to the highest responsible qualified bidder after formal advertising and such other public notice as he deems appropriate: Provided, however, That the Secretary may authorize negotiation of a contract for the disposal of materials if
(1) the contract is for the sale of less than two hundred fifty thousand board-feet of timber; or, if
(2) the contract is for the disposal of materials to be used in connection with a public works improvement program on behalf of a Federal, State or local governmental agency and the public exigency will not permit the delay incident to advertising; or, if
(3) the contract is for the disposal of property for which it is impracticable to obtain competition.
