Not more than $30,000,000 of the amount authorized to be appropriated from the Fund by section 200303 of this title may be obligated by contract during each fiscal year for the acquisition of land, water, or interest in land or water within areas specified in section 200306(a)(2) of this title. The contract may be executed by the head of the department concerned, within limitations prescribed by the Secretary. The contract shall be a contractual obligation of the United States and shall be liquidated with money appropriated from the Fund specifically for liquidation of that contract obligation. No contract may be entered into for the acquisition of property pursuant to this section unless the acquisition is otherwise authorized by Federal law.