Sports Illustrated first reported the letter from Maxwell Blecher. The contents were confirmed by the person who spoke to The Associated Press on the condition of anonymity.
Blecher, a prominent antitrust attorney, acknowledged sending a letter but would not discuss its contents Friday.
NBA Commissioner Adam Silver banned Sterling for life and imposed the fine following the release late last month of a recording in which Sterling makes racist remarks. Silver also called on owners to vote Sterling out of the league.