President Reagan officially gave up the fight Friday for more military aid for the Nicaraguan Contras but with the proviso that if the Sandinistas threaten the rebels, he will call Congress back. A special session is considered unlikely, however. In a statement issued Friday at the White House, Reagan said, "Our policy in Nicaragua remains the same: Freedom for the people of Nicaragua, restoration of democratic institutions, and peace and stability for all of Central America." At the same time, the president made clear what his aides have been indicating all week - that the administration has decided not to make a final attempt to free up military aid for the rebels. White House spokesman Marlin Fitzwater rejected suggestions that the president had written the "last chapter" of his epic efforts to depose the Sandinista government. "The final chapter is not over," Fitzwater said Friday. "The resistance is still there. The president will stay with this to the end."