Luis Picado’s mother remembers the day her son thought he had won the lottery. He came home to their tin-roofed cinder-block house in a Managua, Nicaragua, slum and said he’d found a way to escape poverty and start a new life in the United States. An American man had promised to give Picado, a 23-year-old high school dropout and construction worker, a job and an apartment in New York if he’d donate one of his kidneys.