According to Sky Germany, the clause in Stiller's contract is real and ready to be triggered. Manchester United, Newcastle United and Tottenham Hotspur are all monitoring the 25-year-old, with Real Madrid also linked.
The snag is a small but significant one.
Stuttgart can buy out that release clause themselves for just €2 million. If they do, they are free to set their own price — and Sport Bild reports they would then demand at least €50 million.
That removes the guarantee of a fixed-fee exit. Any club that wants Stiller would then need to negotiate directly with Stuttgart, who have shown no urgency to sell.
Stiller has developed into one of the Bundesliga's best central midfielders since joining Stuttgart from Hoffenheim in 2023. He has registered eight assists in the Bundesliga this season and become a regular Germany international with seven caps.
His impact is not always visible in the statistics. When Stuttgart lost a Bundesliga match to Borussia Dortmund recently, both of the opposition's goals came in the final minutes — moments after Stiller was substituted off in the 88th minute.
The midfielder himself has not closed the door on a move abroad.
"I'm currently not thinking about a move. Generally speaking, I'm interested in moving abroad at some point," he said, adding that Florian Wirtz's switch to Liverpool had caught his eye.
For Manchester United, the appeal is obvious. Casemiro is out of contract this summer and midfield is a priority area. United chief Christopher Vivell is understood to be an admirer of Stiller's profile.
The €40 million clause looks straightforward. Whether Stuttgart allow anyone to use it is another matter entirely.