Ballack has reportedly agreed terms on a two-year contract that will see him rejoin the German Bundesliga side until June 2012 and is expected to confirm the move early next week.
Bayer general manager Wolfgang Holzhauser is keen for the move to be confirmed as soon as possible and has hinted he could withdraw the contract offer if doesn't make a decision soon but Bayer sporting director Rudi Voller insists that Ballack, who is away on holiday at the moment, won't be rushed into any decision.
"By the start of training on Monday we want clarity," Holzhauser said in the Express newspaper.
But Voller said: "We a battling with a host of good clubs for Michael's services. We have good arguments on our sides. I hope he opts for us.
"We will not put Michael under pressure. It certainly does not depend on one or two days."
Bayer, with whom the 33-year-old reached the Champions League final in 2002 before joining Bayern Munich, are keen on acquiring a player of Ballack's experience as they seek to challenge for the Bundesliga title next season, while Wolfsburg are also reportedly interested.
Chelsea last week