U.S. President Trump said he was ready to approve a purchase of the Chinese video-sharing app TikTok, but only if the government received “a lot of money” in exchange.

Microsoft Corp. said it hoped to acquire TikTok's business in the U.S. and three other countries. Mr. Trump said he told the company’s chief executive Satya Nadella that “a very substantial portion of that price is going to have to come into the Treasury of the United States, because we’re making it possible for this deal to happen.”

Legal analysts and others pointed out that the White House had been pushing for a sale of TikTok to U.S. owners, making the demand for payment all the more extraordinary.

“It is completely unorthodox for a President to propose that the U.S. take a cut of a business deal, especially a deal that he has orchestrated. The idea also is probably illegal and unethical,” said Carl Tobias, a law professor at the University of Richmond.

Douglas Irwin, a Dartmouth College economic historian, said that early in President Franklin D. Roosevelt's first term, his advisers debated whether the U.S. should cut barter deals with other governments, but Mr. Roosevelt ultimately rejected the idea and negotiated trade agreements instead. No president since has looked to get so deeply entwined with commercial deals with foreign companies, he said.

Gary Hufbauer, a senior fellow at the Peterson Institute for International Economics, said Mr. Trump’s proposal reminded him of medieval kings who oversaw salt monopolies. “If you wanted to mine some salt, you put money into royal Treasury,” he said.

“It’s a little bit like the landlord-tenant,” the president Trump told reporters at the White House. “Without a lease, the tenant has nothing.” “It’s a great asset,” Mr. Trump said of TikTok. “But it’s not a great asset in the United States unless they have the approval of the United States.” The White House referred questions on how a payment would work to the Treasury Department. A Treasury spokeswoman didn’t respond to a request for comment.