June 15, 1976: Rollie Fingers, A's to Red Sox
Technically this was more of a sale than a trade given that there were no other players going back the other way, but it remains one of the most controversial transactions in baseball history. It didn't last, but there are photos to prove that it went down (that's Fingers being welcomed to Boston by shortstop Rico Petrocelli, above).
With the door to free agency having been opened the previous winter thanks to the arbitrators decision in the Andy Messersmith-Dave McNally case, A's owner Charlie Finley realized he wouldn't be able to keep the core of his team that had won five straight AL West titles from 1971 to '75 (including World Series titles in '72, '73 and '74) together past the end of the ‘76 season. In what Sports Illustrated's Ron Fimrite called "the biggest sale of human flesh in the history of sports," Finley sold Fingers, his top fireman, and outfielder/first baseman Joe Rudi to the Red Sox for $1 million apiece, and ace Vida Blue to the Yankees for $1.5 million.
Commissioner Bowie Kuhn didn’t like Finley or the precedent this set; exercising his "best interest of baseball" powers, he officially voided the sales on June 18, before any of the players had debuted for their new teams. Fingers finished out the year in Oakland, then signed a six-year, $1.6 million free agent deal with the Padres in December. He was elected to the Hall in 1992.