China’s surprise 2 percent devaluation of the yuan on Tuesday pushed the dollar higher and raised the prospect of a new round of currency wars, just as Greece reached a new deal to contain its debt crisis. Stocks fell in Asia and Europe as investors worried about the implications of the move to support China’s slowing economy and exports. China’s move, which the central bank described as a “one-off depreciation” based on a new way of managing the exchange rate that better reflected market forces, pushed the yuan to its lowest against the dollar in almost three years.