At first glance, it appears to be one of the most generous donations in the history of philanthropic giving in America.

A Chinese man has transferred more than 29 percent of HNA Group of China — the equivalent of as much as $18 billion — to a New York-based private foundation. The donation puts him in the same league as donors like Bill Gates and Warren E. Buffett and almost matched the combined giving of all American corporations in 2016.

But it has not been disclosed how that man, Guan Jun, who is in his 30s, came to own such a large piece of one of China’s biggest conglomerates. His registered address in Beijing is a modest apartment at the end of a dingy hallway littered with discarded furniture and bags of trash.

Questions about Mr. Guan, who could not be reached for comment, and the foundation have mounted since HNA disclosed on Monday the donation to the New York-based Hainan Cihang Charity Foundation, which aims to support a variety of causes including anti-poverty efforts. Little is known about Mr. Guan, though he is a director in a company whose address is in the HNA building in Beijing.