The legal construct between gold and silver is a part of the monetary system's problems. There are two ways to handle this:
1. an amendment specifically rewriting that section of the Constitution to eliminate the gold/silver tender problems.
2. rewrite the Constitution in the language of 2015 and update the grammar. This would require re-ratification by all 50 states.
The point being, as the Federal Reserve Note has apparently lost its original connection with gold and silver coin, all payments of debts in Federal Reserve Notes could be declared unconstitutional.
Specifically, Article 1 Section 10:
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.