Nationstar Mortgage Holdings says the deal it announced earlier this month to purchase $63 billion of mortgage servicing rights from Aurora Bank FSB, a subsidiary of Lehman Brothers Bancorp Inc., is part of the servicer’s long-term strategy to drive future growth. Texas-based Nationstar, the home finance unit of Fortress Investment Group, said the purchase represents all of Aurora’s servicing with 75 percent of the loans in non-agency mortgages, comprising approximately $45 billion of Aurora-serviced non-agency loans. Aurora serviced $34 billion of Alt A and negative amortization loans and about $10...