The Secretary may enter into contracts for options to acquire land, water, or interests in land or water within the exterior boundaries of any area the acquisition of which is authorized by law for inclusion in the System. The minimum period of any such option shall be 2 years, and any sums expended for the purchase of an option shall be credited to the purchase price of the area. Not more than $500,000 of the sum authorized to be appropriated from the Fund by section 200303 of this title may be expended by the Secretary in any one fiscal year for the options.