Mortgage giant Fannie Mae this week agreed to pay a hefty $400 million fine and implement a number of remedial measures to settle charges of mismanagement and faulty accounting leveled by the Securities and Exchange Commission and the Office of Federal Housing Enterprise Oversight. In one of the most significant aspects of the settlement, Fannie agreed to limit its retained mortgage portfolio to Dec. 31, 2005, levels, approximately $727 billion. The government-sponsored enterprise can, however,…