Paul Kinzer was planning on waiting until the end of the season, but the Cubs "pursued" earlier negotiations with his client, and Castro wanted to do it, according to the Chicago Tribune:
"If it's a good deal, it's usually a win-win, or both sides feel like they are getting everything they want," Kinzer added on Friday. "So this definitely falls in there."
A possible deal of six or more years would buy out Castro's remaining arbitration and at least two years of free agency.
"(The Cubs) want to wrap up their core of young guys, I'm sure," Kinzer said of the two-time NL All-Star. "So this is the guy they started with."