HONG KONG, Dec 10 (Reuters) - Chairman Hui Ka Yan"s shareholding in embattled China Evergrande Group (3333.HK) has dropped to 59.78% from 61.88%, Hong Kong stock exchange filings showed, in a forced selling by a third party with whom the shares were pledged. The number of shares involved was 277.8 million, worth roughly HK$492 million ($63.08 million) based on the stock"s Friday closing price of HK$1.77. The drop was the result of steps taken Dec. 6-9 to enforce a "security interest" in the shares, the filing said. Reuters could not immediately determine the identity of the entity which sold the pledged shares. Ratings agency Fitch downgraded Evergrande, which has more than $300 billion in liabilities, to "restricted default" on Thursday, after the developer missed a deadline this week to pay coupon payments totalling $82.5 million. read more ($1 = 7.8001 Hong Kong dollars)