The group buying Italian soccer club AC Milan is being led by a little-known Chinese businessman who was sanctioned by the Shanghai Stock Exchange four years ago for failing to disclose a stock sale.

Li Yonghong, chairman of the Sino-Europe Sports Investment Management Changxing Co., was fined 600,000 yuan (about $90,000) by the China Securities Regulatory Commission for failing to report selling $53.6 million in shares of Shanghai Duolun Industry Co., a real-estate and building-materials company, regulatory records show.

When the stock sale was eventually disclosed, Shanghai Duolun’s shares plunged by 7%.

Mr. Li and his representatives didn’t respond to requests for comment. Mr. Li has been out of the public spotlight until his recent bid to buy AC Milan from Fininvest SpA, the family investment vehicle of former Italian Prime Minister Silvio Berlusconi.

The stock-sale violation isn’t expected to affect the pending sale of seven-time European champions AC Milan, according to Fininvest spokesman Simone Finotello, who said Fininvest executives questioned Mr. Li about the sanction. Mr. Li characterized the sanction as a warning, Mr. Finotello said, adding that Mr. Li said he eventually made the sale of stock public.