The costs of surveying, selecting, and conveying lands granted to the Union Pacific Railroad Company shall become due and payable at and on the demand therefor made by the Secretary of the Interior as provided in section
of this title, and nothing in sections
of this title shall be construed or taken in any wise to affect or impair the right of Congress at any time further to alter, amend, or repeal the Act of July 2, 1864, chapter 216, as in the opinion of Congress, justice or the public welfare may require, or to impair or waive any right or remedy in the premises existing on July 10, 1886, in favor of the United States. Sections
of this title shall be subject to alteration, amendment, or repeal.