The issue, Mr. Rousso said, appeared to be an honorarium of $2,000 that he was being paid to participate in the conference. Such payments are allowed for academics visiting the United States, but Mr. Rousso and those involved in securing his release said the customs agents appeared not to realize that at first. “With a tourist visa, I’m not allowed to work,” Mr. Rousso said. “This is true — except for scholars.” The agent who was questioning Mr. Rousso was “concerned that he was giving a lecture and was getting a good stipend to do that,” said Richard J. Golsan, a professor at the university who also had planned to have Mr. Rousso speak to his class last week.

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It was after 1 a.m. Thursday when Mr. Rousso was given back his passport and cellphone, taken to a public area of the airport and told he was free to go. He said he was told that the agent who originally held him was “inexperienced.”

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Mr. Mills said the treatment Mr. Rousso experienced was unusual, but representative of a shift in how some border agents are approaching their jobs. “Now they’re looking really hard for reasons to deny, instead of reasons to admit,” he said.