Timber sale contracts, permits, and other such instruments, authorizing the harvesting of trees or other forest products, with terms of longer than 2 years, shall provide for cancellation in order to prevent serious environmental damage or when they are significantly inconsistent with land management plans adopted or revised in accordance with section 6 of the Forest and Rangeland Renewable Resources Planning Act of 1974, as amended. Such provision shall provide for reasonable compensation to the purchaser for unrecovered costs incurred under the contract and for the difference between the current contract value and the average value of comparable National Forest timber sold during the preceding 6-month period.