When agricultural giant Cargill Inc. put its investment arm up for sale last year, it had all the looks of a simple and quick transaction. Goldman Sachs was in charge of shopping the $10 billion firm, known as CarVal Investors, and powerhouses like Blackstone and Carlyle were among those sniffing around. But in recent months the deal has turned into an embarrassing black eye for Cargill, the largest private company in the U.S.,...