Harris, 23, averaged 14.9 points and made 50.2 percent of his field goals last season, including 42 percent from 3-point range. For his three-year career, Harris has averaged 10.5 points while shooting 46 percent from the field.
Harris would receive $74 million in guaranteed money, with the remaining $10 million through individual and team bonus clauses, according to the sources.
As part of the 2014 draft class, Harris was eligible for his rookie extension. If a deal was not reached by the Oct. 16 deadline, Harris could have become a restricted free agent in July.
According to salaries provided by spotrac.com, Harris moves into the top 10 among the highest-paid shooting guards in the NBA.