Goal.com understands Bayern are leading the chase for Lewandowski, ahead of Manchester United who have made the Poland international a priority summer signing.
|4/1||Robert Lewandowski is 4/1 with BetVictor to sign for Manchester United this summer
"We have reached an agreement with another club and intend to switch clubs this summer," Barthel was quoted as saying by Sport Bild.
"All of Borussia Dortmund's demands will be met."
Lewandowski's agent then went on to stress that BVB have promised the striker that he will be allowed to leave under the right conditions and expects Jurgen Klopp's side to honour their promise.
"There is an interesting offer for Robert that meets Dortmund's criteria and satisfies Robert's demands," he declared. "BVB have assured us that Robert will be allowed to leave under these circumstances. It's now up to the clubs to sort it out."
The 24-year-old has a contract until the summer of 2014, with his exit another potential hammer blow for Dortmund following the news Mario Gotze will join rivals Bayern Munich in the summer.
Speaking to German sport news agency SID, however, BVB general manager Hans-Joachim Watzke denied that the club had accepted a sale to a specific team. "No, that is not true. We don't have any offer of another club," he insisted before adding: "We will talk about everything with Robert."