United have been given permission to speak with the 24-year-old after having a bid accepted by Cardiff earlier this week.
"We're talking to the player's representative," Grayson told the club's official website.
"The talks are ongoing and are progressing. He hasn't had a medical yet, there are other things to be agreed first.
"I don't like talking about things publicly and I don't put timescales on things like this either. Everyone knows that's how I prefer to do things."
Source: PA
Source: PA