The best thing to do is talk with the Chinese Embassy and an attorney in Shanghai.  An attorney located in the U.S. who handles land use matters in China with foreign nationals may be helpful, as well.  The problem is every government is different in terms of what it considers to be proper documentaation for representation purposes (whether it is a letter of representation or power of attorney).  Only an attorney who handles land use matters in China would be helpful. The Chinese Embassy may be able to give you lists of contacts.