It wasn’t the usual kind of Christmas present you’d expect an employee to give his boss at the end of a productive year. Little did the giver know it would be the gift that kept on giving. It was the gift of personal courage that contributed to a future for the national pastime, and marked the end of a custom that was, for all intents and purposes, the last vestige of institutional slavery in the United States.

While today’s attention to the game of baseball is focused on off-season trades — in this the time of the so-called hot stove league — major league baseball marks a milestone on Dec. 24, one that may fly under the radar, but one that truly contributed to something in the DNA of professional sports.

Curtis Charles Flood, the stellar St. Louis Cardinals center fielder and seven-time Gold Glove winner, was to be traded with three other players to the Philadelphia Phillies at the end of the 1969 season.

Flood objected, citing personal and family reasons, as well as reasons pertaining to outside business interests. On Dec. 24, 1969, Flood sent Baseball Commissioner Bowie Kuhn a simply worded letter making his desires clear.

“Dear Mr. Kuhn:

“After 12 years in the major leagues, I do not feel that I am a piece of property to be bought and sold irrespective of my wishes. I believe that any system that produces that result violates my basic rights as a citizen and is inconsistent with the laws of the United States and of the several States.”

“It is my desire to play baseball in 1970, and I am capable of playing. I have received a contract from the Philadelphia club, but I believe I have the right to consider offers from other clubs before making any decisions.

“I therefore request that you make known to all major league clubs my feelings in this matter, and advise them of my availability for the 1970 season.

“Sincerely, Curt Flood.”

The letter — a deft balance of emotion and devastating logic — was one of the first direct challenges of major league baseball’s infamous reserve clause, by which a player was bound body and soul to a team regardless of the player’s wishes, unless that player was traded to another team with absolutely no influence in the matter. It was, for all the pleasant subterfuges of celebrity and income, a benign form of slavery.

It may be hard to understand the importance of Flood’s stand on principle, until you look at the salary structure of major league baseball that persisted for years. In 1946, major league baseball established a minimum player’s salary at $5,000 a year. Some twenty years later, that minimum salary had increased to only $7,000.

Flood’s letter was perhaps the most eloquent cri de coeur of the rights of the undercompensated professional athlete to that time. Flood’s letter was preceded by the demands of Los Angeles Dodger pitchers Sandy Koufax and Don Drysdale, who in 1966 demanded a raise from Dodgers management, were soundly rejected, and who more or less immediately went back to work for the team.

Marvin Miller, the legendary labor organizer who became executive director of the Major League Baseball Players Association, had already helped broker the first collective bargaining agreement in pro sports in 1968.

But Flood backed up his personal demand with personal action. Rather than be forced to play for the Philadelphia Phillies, Flood quit the Cardinals and sat out the 1970 season.

The righteousness of Flood’s cause wasn’t accepted in trials at the district and appellate court levels, where he lost challenges against major league baseball.

Flood ultimately took his case to the U.S. Supreme Court with Miller’s assistance. He would be rejected by the high court, 5-3, in 1972. But the genie was out of the bottle.

The following year, the MLB Players Association extracted concessions from the owners to consent to binding arbitration in future salary disputes.

In the fall of 1975, Dodger pitchers Andy Messersmith and Montreal Expos pitcher Dave McNally declared themselves free agents after playing for a full season without contracts.

And in December 1975, a federal court upheld the arbitrator’s decision that both pitchers could act as free agents, players able to test the waters of the baseball marketplace, effectively dismantling the reserve clause that had prevailed for more than 75 years.

Flood didn’t participate directly in the fruits of free agency; he retired in 1971 after a brief stint with the Washington Senators (now the Texas Rangers). And Flood died in 1997, at the age of 59. He’d no doubt be surprised today at the megacontracts negotiated by players and their agents, for salaries that boggle the mind.

The era of free agency ushered in by Flood’s stand on principle, was a confirmation of the national drive for progress, change and the power of the new. The myriad combination of players now free to pursue their professions wherever they choose have helped grow the game’s potential for surprise, and deepen the joyful unpredictability of the national pastime. You can thank Curt Flood for some of that unpredictability — that possibility made possible when an employee told his boss “I am a man” on Christmas Eve 40 years ago.