Robinson, 50 at the time, addressed the letter to Milton Sacks, a Jewish man who attended Jackie's February 24, 1969 talk at a bank in New Jersey, and Milton wasn't pleased with some of the comments made by the Hall of Famer.

"Being a Jew, I am naturally very sensitive when an attempted genocide which resulted in over 6,000,000 Jews being exterminated in a short period of time is treated very lightly and made to seem insignificant as compared to '140,000,000 Blacks,'" Sacks wrote to Robinson, believing the legendary player downplayed the Holocaust.

Sacks also asked Robinson to provide his source for the statistics on how many Africans died during slavery.

At the time, a local NJ newspaper described Robinson's comments ... "[Jackie] reminded the audience of the mass murders of Jews during World War II, but said that during the slave trading years, an estimated 140 million blacks were killed."

The paper wrote Robinson said ... "That figure was the population of the United States in 1945," adding, "That's something to think about."

Jackie, who clearly didn't agree with the assertion, crafted a handwritten response on "JACKIE ROBINSON" letterhead.

After beginning with a pleasantry, Robinson fired back ... "I was surprised by your sensitivity. How can you say 6,000,000 murdered Jews seemed insignificant regardless of the number of slaves killed."

Jackie continued ... "What difference does a number make one would have been too many and it seems to me you are overly sensitive and it does surprise me. I resent your inference and usually say to 'hell' with individuals who feel as you do about the fact 140,000,000 blacks were killed."

Robinson then invited Sacks to call him on the phone the following week so he could reveal his sources. It's unclear if the conversation ever occurred.

Milton's typed letter to Jackie is also included in the sale.

Robinson, a 6-time All-Star and 1949 NL MVP, is one of the most important figures in the history of American sports. The second baseman broke MLB's color barrier in 1947.