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 anything 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.
What two people contract amongst each other ought not be impaired by the state.
You're misinterpreting Art. I § 10, which is referring to the (currently 50) states. Only Congress may "impair[] the obligation of contracts." A legislative action doing such a thing is, for example, bankruptcy. Even with respect to the states, the prohibition has never been absolute, and certainly is not now.
This section of the Constitution was drafted to prevent practices in which the states would either pass "private bills" relieving (usually) some wealthy person of his contractual obligations, or basically pass laws giving the property of foreigners to colonists, causing a fear by the Framers that this would scare away foreign capital.
Check
Federalist No. 10 by Madison for a fuller explanation.
ETA: Incidentally, not arguing your actual point, at least not here, just your citation.
Thanks for raising this. Yes, little in the Constitution is absolute. So much nuance and reinterpretations....
Its not so much a misinterpretation if you look at the question asked, though agree that it is very much a stretch.
Poster was looking for some general
constitutional support for putting the breaks on this sort of FinCEN abuse, and there isn't much there on this matter.
So closest is Article 1 § 10, (which arguably limits the state part of FinCEN, even though congress authorized the Fed to delegate this to states),
Or 10th amendment, (which takes significantly more political action, rather than legal action)
This particular incident would not be one I'd pick to take to the supremes though, and constitutional matters end up there.