Tupac Shakur suggested to Madonna he broke up with her because of race, in an emotional letter attributed to the doomed rapper.
The 1995 missive, addressed to “M”, said being with a black man could only help her career, but that he might let down his fans.
Madonna confirmed two years ago they had had a relationship, though it is unclear how long it lasted.
The letter is up for auction with a starting bid of $100,000 (£77,000).
Dated 15 January 1995, it was penned while Tupac was serving a prison sentence for sexual assault and 18 months before he was shot dead. Both artists were then at the height of their fame.
“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, then 23, wrote from New York’s Clinton Correctional Facility.
“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.
“Like you said, I haven’t been the kind of friend I know I am capable of being,” he wrote, adding: “I never meant to hurt you.”
Rolling Stone magazine said it had confirmed the authenticity of the document, which was first published by TMZ.
Tupac – whose parents were both Black Panthers – also suggests Madonna, then 36, hurt him by saying in an interview that she was “‘off to rehabilitate all the rappers and basketball players’ or something to that effect”.
“Those words cut me deep seeing how I had never known you to be with any rappers besides myself,” he wrote.
“It was at this moment out of hurt and a natural instinct to strike back and defend my heart and ego that I said a lot of things.”
He adds: “Please understand my previous position as that of a young man with limited experience with a extremely famous sex symbol.”
Tupac concluded: “It’s funny but this experience has taught me to not take time for granted.” He signed off with a heart symbol.
On 7 September 1996, the rapper – who sold over 75 million records worldwide – died in a drive-by shooting in Las Vegas after watching a Mike Tyson boxing match.
The letter will be up for auction at the Gotta Have Rock and Roll sale, which is scheduled for 19 – 28 July.