The 24-year-old, who was linked with Manchester United last summer, is reportedly on Bayern's radar following his impressive performances for the defending German champions over the past two-and-a-half years.
"I was on the road this week," Matthaus told Sky Bundesliga. "I heard that Lewandowski has an agreement with Bayern. Only the date of the move remains undecided.
"Lewandowski's contract runs out in 2014 and Dortmund will only get a fee in 2013 - surely a high fee.
"I heard it was something between €25m-€30m, otherwise he will go to Bayern for free next year."
Lewandowski joined Dortmund from Lech Poznan in 2010.