No agreement permitting a third party to acquire the rights of a purchaser under a timber sale contract may be recognized and approved by the Forest Service except in writing, signed by the officer approving the sale, his successor, or superior officer. Such approval shall not relieve the purchaser of his responsibilities or liabilities under the timber sale contract and may be given only if—

(a) The third party is acceptable to the Forest Service as a purchaser of timber under the conditions and requirements then in effect for similar timber sales and assumes in writing all of the obligations to the Forest Service under the terms of the timber sale contract as to the uncompleted portion thereof, or

(b) The rights are acquired in trust as security and subject to such conditions as may be necessary for the protection of the public interests.