A contract in form approved by the Commissioner shall be signed by the purchaser at the time of sale and executed on behalf of the United States by the Commissioner. A copy of the contract shall be furnished to the appropriate land office of the Bureau of Land Management for entering in the tract books. The contract shall contain a description of the land to be sold, the price and terms of sale, a full statement by the purchaser respecting his qualifications, including citizenship, a description by the purchaser of his present holdings, and a statement by him of the irrigable acreage of those holdings. The contract shall also contain a statement by the purchaser with respect to his knowledge as to whether the land is mineral or non-mineral in character, as well as all appropriate reservations, mineral and otherwise, required by law to be made on entries and patents. Assignments of contracts may be made only with the consent of the Commissioner and to persons legally qualified to be purchasers.