Ferguson is on loan at the League One side and has been all season long, but Harris is unclear whether Newcastle still have plans for him.
He has put this to the test with an offer to sign the 19-time capped Northern Ireland international permanently.
He said: "We’ve gone really strong to Newcastle and made a bid for his services. We’re just waiting in the next few days to see how that unfolds.
"The board are backing me in trying to buy him. Fergie showed his quality here. He’s done really well for us and there’s so much more to come. We’re working hard with Ferguson and working hard with Newcastle to make a deal happen."
Ferguson's contract at Newcastle expires in June, but for Millwall and the player it is best for him to get his future sorted out heading into the second half of the campaign one way or another.
The 24-year-old has made 22 League One appearances for Millwall this season, scoring two goals and providing three assists.
Last season he was loaned out to Rangers but barely featured due to injury, playing only twice. He has made 32 first team appearances for Newcastle United.