Over the past dozen years, Sanford Solny has built a New York real estate empire, snatching up small residential buildings across the city that churn out hundreds of thousands of dollars in rent. His portfolio would be enviable but for one thing — much of it, prosecutors and homeowners contend, was stolen. In criminal charges and lawsuits, they have accused him of fraud: offering to help homeowners facing foreclosure by arranging to pay off their mortgages, while actually tricking them into signing over their buildings at bargain-basement prices. In nearly every case, the mortgage was never paid, leaving the homeowner with no property but a pile of debt. The practice…