This financial windfall stems from a deal involving Taylor Harwood-Bellis, who joined Southampton on loan last summer with a clause that would make his move permanent upon their promotion.
The Saints defeated Leeds United 1-0 in the Championship Playoff final at Wembley Stadium over the weekend, securing their immediate return to the top flight after being relegated in the 2022-23 season.
The 22-year-old previously had loan spells with Blackburn, Burnley, Stoke and Belgian Pro League outfit Anderlecht.
Southampton promotion triggers £20m transfer clause for Harwood-Bellis
According to talkSPORT, Southampton's promotion means Harwood-Bellis' loan will now become permanent under the terms of the original deal made with Manchester City last summer.
The report adds that the Saints' victory at Wembley entails they will pay the Premier League champions £20m for the center-back. These funds are expected to assist Pep Guardiola in reinforcing his squad for their title defence in the upcoming season.
Harwood-Bellis becomes the fifth player in the last two years to make the move from the Etihad Stadium to St Mary's, following the likes of Gavin Bazunu, Juan Larios, Samuel Edozie and Shea Charles.
Throughout the season, the central defender has been a consistent presence for the Saints, featuring in 46 matches and netting two goals across all competitions.
This marks the second consecutive season in which Harwood-Bellis has secured promotion to the Premier League, following his loan spell with Vincent Kompany's title-winning Burnley side of 2022-23.
Harwood-Bellis completes permanent move to Southampton
Following their Playoff victory, the defender expressed his immense joy at officially joining Southampton.
Speaking to BBC Sport, Harwood-Bellis said, "I can't put it into words, it's different because I know I'm a Southampton player now".
"I wanted to stay here and to know that we'll be in the Premier League and there's not even a discussion to happen now [with Man City] and to just turn up for pre-season, I can't describe that feeling."
The 22-year-old initially joined City's academy in 2008 and made his debut for the first team at the age of 17 in 2019. During his time with the club, he made eight appearances and scored one goal.