In a note to Madonna, reportedly written by Shakur while he was serving time in prison in 1995, he acknowledges ending their romantic relationship because of her race.
"For you to be seen with a black man wouldn't in any way jeopardize your career, if anything it would make you seem that much more open and exciting," Tupac wrote. "But for me at least in my previous perception I felt due to my 'image' that I would be letting down half of the people who made me what I thought I was."
Gotta Have Rock and Roll is auctioning the letter later this month.
While CNN has not independently confirmed the letter's authenticity, the auction site allowed TMZ to photograph the note and publish excerpts.
Shakur apologized to Madonna in the letter, adding "I never meant to hurt you."
Madonna has not commented publicly about Shakur's alleged remarks, but spoke about her relationship with him in a 2015 interview with Howard Stern.
The singer said Shakur was partly responsible for her expletive-laden appearance on "The Late Show" in 1994.
"He got me all riled up about life in general, so when I went on the show I was feeling very gangster," she said.
Shakur was shot and killed in 1996.
Last month, Madonna posted a throwback photo of herself with Shakur in honor of what would have been his birthday.
"Happy Birthday Tupac," she wrote in the caption. "Not sure what my dumb a** facial expression is all about. Maybe we were at the circus."