Bundesbauer is correct. While the Federalist Papers make clear that fiat money was to be feared, and the Constitution does prevent states from dealing in it, there is no concrete restriction in the constitution itself which prevents the federal government from engaging in paper currency. The keywords here are federal and currency. The federal government is given the power to 'coin' money which obviously implies that it should be of a metallic nature, however, it doesn't specify which type of metal it should be nor does it say anything about prohibiting the creation of fiat currency. This is a weakness in the constitution. The only mention of hard money is in the section you cited, but that is a restriction put on the states and not the federal government.
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