BVB are in talks to sign the veteran and Barber admits they won't seek to block the move.
"Of course we would like to keep him. But we are also thinking about the phase he is in in his career and the opportunity that Dortmund could offer him," said Barber.
He also told talkSPORT: "Ever since we set up camp in the Premier League, Pascal has been a fantastic servant to this club.
"Right up until the end, he was still scoring goals and providing assists. He played his way into the German national team and made it into the squad for the European Championships."
BVB chief Lars Ricken has also stated in the last 24 hours: "I can't say anything final about Pascal Gross yet. He's not our player either. That's why I find it difficult to talk about him."