If Giancarlo Stanton opts out of his new 13-year, $325 million contract with the Miami Marlins after the first six seasons, he'll be walking away from a staggering $218 million over the final seven seasons, sources familiar with the deal told ESPN.com Tuesday.

However, one source said Stanton was so motivated to give his team the financial flexibility to win now, he agreed to a heavily backloaded contract structure that will pay him just $30 million over the first three seasons.

According to a major league source who had seen the terms, Stanton's salaries over those first three seasons will be only $6.5 million in 2015, $9 million in 2016 and $14.5 million in 2017, far less than he could have earned through arbitration in 2015 and 2016 and then via free agency. He would earn $77 million over the next three seasons and could opt out of the contract after 2020, following his age 30 season.

So the Marlins would be on the hook for only $107 million of the deal over the first six seasons, which computes to an average annual value of just $17.83 million per season before Stanton would have the right to exercise the opt-out clause.

If Stanton remains a Marlin, the $218 million he would collect over those final seven seasons would average out to $31.14 million a year. He will have a complete no-trade clause through the life of the contract, a first for the Marlins under the ownership of Jeffrey Loria.

If Giancarlo Stanton opts out of his new deal after the first six seasons, he'll be walking away from $218 million over the final seven seasons, sources said. AP Photo/Wilfredo Lee

Sources said the contract was structured in a way that reflects Stanton's desire to give the Marlins the financial flexibility to construct a winning team around him in the early seasons of the deal. If they don't use that flexibility to add the pieces to build a contender, he could walk away and become a free agent again at age 31.

However, he would also be walking away from the majority of his guaranteed money by doing so.

The Marlins are looking for a middle-of-the-order bat to hit behind Stanton, and they have shown aggressive interest in free-agent first baseman Adam LaRoche, among others.

The $325 million deal with Stanton, who turned 25 this month, represents the largest contract in the history of North American professional sports. The Marlins have scheduled an 11 a.m. ET news conference Wednesday.