The midfielder, whose current deal expires in 2015, has been targeted by Old Trafford and die Roten's transfer chief fears he may not renew his contract.
The 24-year-old’s current deal with the European champion expires in June 2015, while David Moyes was pictured speaking with the midfielder’s agent during a scouting trip in January.
Man Utd interest in Kroos in January was not enough to trigger a bid for the Bayern star, who refused to commit his future to Pep Guardiola’s side, but the Roten chief says they will not sell Kroos on regardless of whether or not he signs.
"We won't sell Toni before his contract ends," Sammer told Suddeutschen Zeitung. "I'll repeat what Uli Hoeness said: Bayern are a buying club, not a selling club.
"We all want him to extend his deal with us, there is no doubt about that. But he's not a simple case. At the moment we can't say the chances that he'll stay are 80-20 or 70-30.
"Sometimes Toni still thinks too much about his own performance, but he has the ability to eventually take responsibility for our entire team's performance."