The typical southron complaint. Something is unconstitutional because you say it is.
No, the Constitution gives the government the authority to standardize the weight of and coin money, not 'regulate the value of the dollar'.
Article I, Section 8, Clause 5: "To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;" I thought you read it?
REG'ULATE, v.t.
1. To adjust by rule, method or established mode; as, to regulate weights and measures; to regulate the assize of bread; to regulate our moral conduct by the laws of God and of society; to regulate our manners by the customary forms.
http://65.66.134.201/cgi-bin/webster/webster.exe?search_for_texts_web1828=regulate
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Congress can decide what the standard weight of the coin will be, NOT the value of what it's worth.