Banks seeking to clean up their balance sheets by selling U.S. mortgages made during the real- estate lending boom are encountering documentation problems, Cantor Fitzgerald LP said. In some cases faulty files are lowering loan prices or extending the time it takes to complete sales, said Jason Kopcak, the head of whole-loan trading at the New York-based broker. Residential and commercial mortgages owned by banks looking to sell often lack the papers required by buyers, including documents needed to foreclose, Kopcak said.