The man, Patrick Campbell, 33, from Sierra Leone, was arrested on charges of knowingly brokering goods destined for Iran, according to a criminal complaint filed in United States District Court for the Southern District of Florida in Fort Lauderdale, where he was to meet the buyer. United States’ sanctions against Iran prohibit exporting most goods from the United States to Iran.

Mr. Campbell was being held in New York City on Friday, pending a hearing.

A law enforcement official who was briefed on the matter, but was not authorized to speak publicly on the matter, said the material in Mr. Campbell’s shoes was not enriched uranium. But it was unclear whether more tests had been done to determine exactly what Mr. Campbell had brought into the United States.

Yellowcake featured prominently in the run-up to the Iraq war. Information about Iraq’s desire to acquire it from Niger was used by the Bush administration to help justify the invasion, notably by President George W. Bush in his State of the Union address in January 2003. But the information was later revealed to have been based on forgeries.

In this case, which was first reported on the Smoking Gun Web site, the undercover Homeland Security agent was skeptical of Mr. Campbell’s ability to deliver, according to court documents.

Their communications began soon after the agent posted an advertisement on alibaba.com, an e-commerce company based in China, in the spring of 2012. The agent claimed to be “an American broker representing the interests of individuals in Iran who were seeking to purchase Uranium 308,” according to the criminal complaint, which used an incorrect term to refer to the yellowcake.