The national housing slowdown is testing the theory that the Manhattan market is uniquely positioned to remain strong no matter what happens around the country. A Corcoran Group report showed a 9.9 percent drop in September sales compared to last year. “I certainly don’t think Manhattan is recession-proof,” said Christopher Mayer, director of Columbia University’s real estate center. Nouriel Roubini, an NYU economics professor, says prices will fall 10 percent over the next two years. So far this quarter, 1,473 sales have been recorded in Manhattan, compared with 4,337 for the entire quarter last year, according to Terra Holdings, which owns brokerages Brown Harris Stevens and Halstead Property.