BY PRUDENCE HO AND YVONNE LEE

HONG KONG—American International Group Inc.’s sale of part of its stake in Asian life insurer AIA Group Ltd. raised the expected US$6 billion, but shares in the Hong Kong-listed unit slumped in the wake of the deal.

AIA shares, which were suspended from trading Monday, closed at HK$26.75, below the offer price of HK$27.15, already a 7% discount to Friday’s closing price of HK$29.20. The benchmark Hang Seng Index fell 2.2%, in part on China’s cut Monday to its growth forecast.

AIG, a New York-based insurer, said it sold 1.72 billion shares, or …