WASHINGTON — The Justice Department has concluded that the Obama administration can lawfully pay the community group Acorn for services provided under contracts signed before Congress banned the government from providing money to the group. The department’s conclusion, laid out in a recently disclosed five-page memorandum from David Barron, the acting assistant attorney general for the Office of Legal Counsel, adds a new wrinkle to a sharp political debate over the antipoverty group’s activities and recent efforts to distance the government from it. Since 1994, Acorn, which stands for the Association of Community Organizations for Reform Now, has received about...