((a)) Notwithstanding the provisions of existing National Forest timber sale contracts that are directly affected by conveyances authorized by this chapter, the Secretary of Agriculture is authorized to modify any such contract, with the consent of the purchaser, by substituting, to the extent practicable, timber on other national forest lands approximately equal in volume, species, grade, and accessibility for timber standing on any land affected by such conveyances, and, on request of the appropriate Village Corporation the Secretary of Agriculture is directed to make such substitution to the extent it is permitted by the timber sale contract without the consent of the purchaser.
((b)) No land conveyed to a Native Corporation pursuant to this chapter or by operation of the Alaska National Interest Lands Conservation Act which is within a contingency area designated in a timber sale contract let by the United States shall thereafter be subject to such contract or to entry or timbering by the contractor. Until a Native Corporation has received conveyances to all of the land to which it is entitled to receive under the appropriate section or subsection of this chapter, for which the land was withdrawn or selected, no land in such a contingency area that has been withdrawn and selected, or selected, by such Corporation under this chapter shall be entered by the timber contractor and no timber shall be cut thereon, except by agreement with such Corporation. For purposes of this subsection, the term “contingency area” means any area specified in a timber sale contract as an area from which the timber contractor may harvest timber if the volume of timber specified in the contract cannot be obtained from one or more areas definitely designated for timbering in the contract.