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"The club have done their homework, Julian has a binding contract with us until June 30, 2019," Hoffenheim's billionaire backer Dietmar Hopp told supporters at the club's annual general meeting on Monday.
"Julian is an exceptional coach. We'll have to clone him sometime, but the clone should be ready by July 1, 2019."
Hopp's joke indirectly confirms reports that Nagelsmann, Germany's coach of the year in 2016, has a buy-out clause, said to be worth five million euros ($5.8m), from 2019.
In June, Hoffenheim announced they had extended his contract until June 2021, but he seems increasingly unlikely to see it through.
In February 2016, Nagelsmann, then just 29, became the youngest coach in Bundesliga history.
He become a star in Germany by pulling Hoffenheim back from the brink of relegation in 2015/16 to then finish fourth in the Bundesliga last season.
Reports are rife that Dortmund want Nagelsmann for next season and Bayern Munich are also said to be interested.
On Sunday, Dortmund named Peter Stoeger as head coach after Dutchman Peter Bosz was sacked on Saturday with the team winless in their last eight league games.
Stoeger, who has a contract until June, is reportedly an interim measure before Nagelsmann arrived for next season, but Hopp will fight to keep him.
"The demand for Julian is damned high and increases with every success - he's just too good," Hopp said.
"But we still have a year and a half left, so in the meantime, we want to continue celebrating with him and we want to push the celebrations even more."
Nagelsmann was in the audience listening to Hopp speak, but is clearly annoyed by constant questions over his future.
"Things aren't changing every two days and two days ago, I said that I have had no contact (with Dortmund)," said Nagelsmann on Sunday after Hoffenheim's 2-0 defeat at Hanover left then sixth in the table.
