While his father, Joseph R. Biden Jr., was vice president, Hunter Biden began developing relationships that led to an audacious proposal to sell the land around the United States Embassy building in Romania to a group that included a Chinese company.

Hunter Biden was involved in the proposed deal from multiple perspectives, creating what he privately acknowledged to an associate was an ethical quagmire, according to documents and four people with knowledge of the matter who were not authorized to speak about it.

Even after years of investigations, and a suggestion by federal prosecutors that Mr. Biden’s work in Romania left him vulnerable to a foreign lobbying charge, the full story of his efforts there has not been told before. Previously unreported details exemplify how Mr. Biden’s pursuit of lucrative foreign deals fit into the broader foreign influence industry and complicated U.S. diplomacy.

The deal he was pursuing left open the possibility that the Chinese could have ended up with an ownership stake in a critical asset — the land around the embassy in Bucharest, and possibly the land on which the embassy sat. The possibility concerned one of his former partners who sought to avoid the outcome at the time, while another partner later rejected the idea that it was ever on the table.

The chapter fits into a well-worn playbook used by deep-pocketed foreign interests to seek favor in Washington by courting politically connected Americans, including presidential family members like the adult sons of President Trump or the son and brother of former President Jimmy Carter.

Hunter Biden’s dealings in Romania began when his father was vice president, but the land deal did not take shape until the elder Mr. Biden left that office. Some of Hunter Biden’s foreign dealings attracted wide attention when his father ran for president in 2020, and became a major political and legal issue during the Biden presidency, though the Romania effort remains relatively unknown.