The reasons had been building for months, but the clincher came when the collector, fertilizer magnate Dmitry Rybolovlev, resold the “Salvator Mundi” for $450 million during last year’s fall art auction season, more than triple the amount he paid, one of the people said. Had the case proceeded, Rybolovlev’s windfall could have enabled the defense to claim that he wasn’t a fraud victim because he profited in the end.