MIAMI—At the height of the real-estate boom in 2006, Ugo Colombo, an Italian-born developer, bought a stake in an empty plot of land along the Miami River, facing Biscayne Bay, in a deal that valued the site at roughly $25 million. Last week, Mr. Colombo and his partners sold the same piece of land, now one of the last undeveloped waterfront parcels in downtown Miami, to an Argentine grocery-store magnate for $125 million.