The exact terms of that obligation have not been made public, with West Ham United having previously asked interested clubs to pay a £25m fee for the DR Congo international outright.
Aston Villa took a similar approach last season with Liverpool midfielder Harvey Elliott, signing him on loan with an obligation to buy for £35m, though that deal was conditional on Elliott playing 10 league games.
He featured in just four and returned to Liverpool this summer, a reminder of how these structured obligations do not always convert into permanent transfers.
Wan-Bissaka missed West Ham's first two Championship matches of the season while the move was finalised. He joined the Hammers on a seven-year contract from Manchester United in 2024 and made 65 appearances across two seasons at the London Stadium, including 25 last term as West Ham were relegated.
Before that, he played 190 times for United, winning the League Cup in 2023 and the FA Cup in 2024, and started all four of DR Congo's matches at this summer's World Cup as they reached the round of 32.
Speaking after the move, Wan-Bissaka said the club's ambition had drawn him to Villa Park.
"I'm excited. I'm happy to be here and be part of this great club. The ambition attracted me. You know the direction the club is going in, and obviously the opportunity to play at this level. His aim for the season and what he's done over the last few years for the club, that's been motivating," he said, referring to manager Unai Emery.
With a possible debut against Brighton this weekend, the 28-year-old is looking forward to experiencing Villa Park as a home player for the first time.
"It'll be a new experience, getting to know the players and the fans, and also the atmosphere," he said. "Every game will be a challenge, but I'm looking forward to it. My aim is just to have a successful season, also add trophies to that as well."
Asked for a message to supporters, he added: "Just to get behind us, support us as usual, and we will give it our all."