By Bob Davis, Alex Leary and Kate Davidson

WASHINGTON -- President Trump said he was ready to approve a purchase of the U.S. operations of the Chinese video-sharing app TikTok, but only if the government received "a lot of money" in exchange -- an assertion of presidential power that appeared to lack precedent.

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.

The president, a former New York real-estate developer, had a different analogy in mind -- "key money," an extra fee paid to secure a hard-to-get property. "It's a little bit like the landlord-tenant," the president told reporters Monday 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."

Later in the day, he was asked to clarify his remarks. "It would come from the sale," Mr. Trump said. "Whatever the number is, it would come from the sale. Which nobody else would be thinking about but me. But that's the way I think. And I think it's very fair."

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.

Mr. Trump has drawn often to his former life as a developer, presenting himself as a master negotiator on everything from international trade accords to dealings with North Korean leader Kim Jong Un. The president's comments also amplify how deeply he involves himself in trade and investment decisions, which economic historians also say is a stark departure from the past.

While the U.S. government for decades has analyzed foreign investments in the U.S. to see whether they could create national-security problems, the decisions are usually left to the members of a secretive interagency group called the Committee on Foreign Investment in the U.S.

Under Mr. Trump, the U.S. has taken a hard line on China through Cfius. Last year, Cfius ordered a Chinese company to sell gay-dating app Grindr, citing the risk that the personal data it collects could be exploited by Beijing.

Mr. Trump and his national-security team cited similar concerns about TikTok, raising the prospect that its Chinese owner, Beijing-based ByteDance Ltd., could be forced to share data it collects on American users with the Chinese government.

With TikTok, however, the president's requirement for a payment showed a much greater involvement. It follows two years of personal attention to the trade battle with China, including deciding how and when to assess tariffs -- and when to back off and cut a deal.

"This is an extension of Trump's generalized view that he can micromanage the industrial sphere," said Mr. Hufbauer, the Peterson trade expert.

Tony Fratto, a former George W. Bush Treasury official and partner at the Washington public-affairs firm Hamilton Place Strategies, said it was unlikely any payment would stand up.

"There is no situation where either Microsoft or the Chinese or TikTok or Bytedance will be sending a check to the U.S. government, except in the normal course of their regular tax obligations," Mr. Fratto said.

The president has made other broad assertions of presidential power, which may be popular with his political base, only to back off later. Last August, at the height of the U.S.-China trade battle, he tweeted that he "hereby ordered (U.S. companies) to immediately start l ooking for an alternative to China, including bringing your companies HOME." Nothing came of that.

Earlier this year, Mr. Trump said he had the power to force states to reopen their economies amid the coronavirus pandemic, though he backed off that claim, too. He has recently pointed to a Supreme Court decision on an immigration program for young undocumented residents, arguing that it gave him authority to make more sweeping changes without Congress. He cited the same decision as grounds for a healthcare overhaul, though he has yet to offer one.

"Trump often tries to prove how strong he is by taking novel and extreme positions," said Alex Conant, a Republican strategist in Washington. "A lot of people in Washington may roll their eyes at Trump's off-the-wall proposals, but we shouldn't underestimate their simplistic appeal to many voters."

The comments, though, can make life tough for companies, which don't want problems in Washington.

In this case, Microsoft declined to comment beyond its statement released Sunday night in a blog post: Microsoft said it was "committed to acquiring TikTok subject to a complete security review and providing proper economic benefits to the United States, including the United States Treasury."

It wasn't known whether Microsoft was talking about the taxes it would pay or some other arrangement.

Mr. Trump indicated a deadline of Sept. 15, after which TikTok would be banned in the U.S. Microsoft said Sunday that it would move quickly to pursue discussions with ByteDance and it aims to complete the negotiations by Sept. 15.

TikTok says it has 100 million users in the U.S. In a statement posted Saturday on its platform, Vanessa Pappas, TikTok's U.S. general manager, assured users that it plans to continue operations.

--Aaron Tilley contributed to this article.

Write to Bob Davis at bob.davis@wsj.com, Alex Leary at alex.leary@wsj.com and Kate Davidson at kate.davidson@wsj.com

Corrections & Amplifications

This item was corrected on August 4, 2020 to reflect that President Trump said he was ready to approve a purchase of the U.S. operations of the Chinese video-sharing app TikTok. The original version incorrectly implied that all of TikTok was for sale.