A Manhattan man said that $8,400 was stolen from his backpack at the Y.M.C.A. East Hampton RECenter on Friday evening.Max Nuti, who is 47, called East Hampton Village police after he realized the money was missing. He told officers at 10:54 p.m. that he had left his backpack unattended on a bench in the men’s locker room between 5:45 and 6:15 p.m. There was $8,000 in $100 bills inside it, and $400 cash in a wallet, according to a police report.Mr. Nuti said he also filed a report with the staff at the Y.M.C.A. Chief Gerard Larsen said detectives are handling the case.