The Treasury Department, in a statement, said Mr. Ahmad used his art as a place to shelter money “in a pre-emptive attempt to mitigate the effects of U.S. sanctions.” It said he also set up a gallery in Beirut, the Artual Gallery, as a front to launder money. The department declined to provide details on how that process worked. The gallery, which is run by his daughter, also focuses on American art.

There has long been a debate in the art world about how susceptible the trade is to money laundering. It has obvious vulnerabilities: Transactions are largely unregulated, and the identities of buyers and sellers are typically not publicly disclosed. The value of art is also subjective, and easy to inflate or deflate, which can create a path to move large amounts of money. Buy a painting for $100,000 today and sell it for $1 million tomorrow, and you’ve just moved $900,000.

But many in the art world have strongly denied that illicit deals take place, insisting that a dearth of examples show that the perception of the art market as a place infested by a shadowy underworld is just a myth.

The Treasury Department disagrees. Marshall Billingslea, assistant secretary for terrorist financing, called the buying and selling of art a “well understood method of illicit financial transactions.”

In this situation, Mr. Billingslea said, “we have a Hezbollah financier who is converting a significant amount of his ill-gotten gains through the blood diamond trade into high-value art, that in turn creates a lack of transparency in the transactional process,” he said. “It’s a way of getting around the formal financial system.”