16 U.S. Code § 1f - Challenge cost-share agreement authority
For purposes of this subsection:
(A) The term “challenge cost-share agreement” means any agreement entered into between the Secretary and any cooperator for the purpose of sharing costs or services in carrying out authorized functions and responsibilities of the Secretary of the Interior with respect to any unit or program of the National Park System (as defined in section 1c (a) of this title), any affiliated area, or any designated National Scenic or Historic Trail.
(2) Challenge cost-share agreements
The Secretary of the Interior is authorized to negotiate and enter into challenge cost-share agreements with cooperators.
(3) Use of Federal funds
In carrying out challenge cost-share agreements, the Secretary of the Interior is authorized to provide the Federal funding share from any funds available to the National Park Service.
(4) Available funds
Out of any amounts in the Treasury not otherwise appropriated, $20,000,000 shall be made available to the Secretary of the Interior for fiscal year 2018, and $30,000,000 shall be made available to the Secretary of the Interior for fiscal year 2019, without further appropriation and to remain available until expended, to pay the Federal funding share of challenge cost-share agreements for deferred maintenance projects and to correct deficiencies in National Park Service infrastructure.
(5) Cost-share requirement
Not less than 50 percent of the total cost of project  for funds made available under paragraph (4) to pay the Federal funding share shall be derived from non-Federal sources, including in-kind contribution of goods and services fairly valued.
 So in original.