Sharp is one of the most coveted players outside the Premiership, with Norwich City also making a bid - believed to be £2.5million - over the Bank Holiday weekend.
United's asking price is for £2m up front, plus various add-ons. So far Preston North End and Sheffield Wednesday have been priced out of the market.
Wednesday boss Brian Laws, who managed Sharp at Glanford Park, has ruled out another approach from the Owls.
"If there's a bidding war forget it, we'll be out of it like a shot," he said.
"Two million pounds for a player that I know, I know what he's capable of, that's too high.
At this moment in time we made an offer for him, they turned it down and that is it at the moment. We have to look at the player's true, true value.
"Sharp is an exceptionally good player but unproven in the Championship, yet I believe he would do an exceptionally good job in the Championship and beyond."