Berbatov spent five years with Leverkusen before moving to the Premier League with Tottenham Hotspur in 2006 and on to United two years later.
However, his future at Old Trafford is uncertain as he has started only two Premier League games this season after being the division's joint top scorer last term.
His current contract expires at the end of the season, although United do have an option of a further 12 months and Sir Alex Ferguson recently admitted the club were likely to exercise that deal.
The rumours concerning a January exit, though, have not gone away and the Bulgarian's representatives are waiting for United to make a decision regarding Berbatov's contract.
A formal approach from Leverkusen has yet to be made, but Berbatov retains affection for a club he helped to reach the 2002 UEFA Champions League final at United's expense.
"It's more than a rumour," Ivan Dobrinov, a member of the group which represents Berbatov, told Bild. "Leverkusen have it on their minds to get Berbatov.
"There is contact, but nothing concrete yet. It all depends on United's plans.
"We wait until 2nd January. Until then, United have the option but if that does not happen then Berbatov would be a free transfer in the summer.
"The player has never forgotten Leverkusen, the club is still very dear to him. Furthermore, he has a good relationship with the leadership at Bayer."
Leverkusen CEO Wolfgang Holzhauser echoed Dobrinov's comments and is also waiting to see whether the Red Devils decide to keep the 30-year-old.
Holzhauser said: "I would even say that the contact is friendly. But there is certainly nothing concrete at the moment.
"I also read that Manchester United would like to extend his contract."