The three-coin - Gold ($5), Silver ($1) and Copper-Nickel (CuNi) Clad ($0.50) - commemorative program to mark the 50th Anniversary of the 1941 completion of the Mount Rushmore National Memorial, was signed into law on July 16, 1990 by US President George H(erbert) W(alker) Bush. The bill was introduced by Tim Johnson (D-SD) in February 1989. (101st Congress)
View of Completed Mount Rushmore National Memorial
(Image Credit: Library of Congress, Prints and Photographs Division. Public Domain.)The Act called for collected surcharges ($35 / $7 / $1) that were to be split 50/50 between (1) Reducing the National Debt, and (2) the Mount Rushmore National Memorial Society "to assist the Society's efforts to improve, enlarge, and renovate the Mount Rushmore National Memorial."
In the 102nd and 103rd Congresses, multiple attempts were made to alter the original bill's surcharge distribution model - amendments were introduced in the House of Representatives ("House") and Senate. The amended language called for the first $18,750,000 in collected surcharge funds to be paid to the Society with any amounts above such threshold to go toward reducing the National Debt.
None of the four standalone amendment bills in either the 102nd or 103rd Congresses gained the support needed for approval, but the Riegle-Neal Interstate Banking and Branching Efficiency Act of 1994 (103rd Congress) was amended to include the surcharge model change; it became Public Law on September 29, 1994.. The bill/Act included language for retroactive implementation to address any funds already distributed.
While it sold over 1.9 million coins, the Mount Rushmore coin program's generated surcharges from these sales fell short of the $18.75 million threshold - they totaled ~$12.1 million. The collected surcharge funds, therefore, were all paid to the Society - the potential for National Debt reduction went unrealized.
1991 Mount Rushmore National Memorial Golden Anniversary - Silver Dollar

For more of my topics on commemorative coins and medals, including more on the history and design of the Mount Rushmore commemorative coin program discussed here, see:
Commems Collection.