Reports in Scotland suggest the fee for Hayes will stand in the region of €1.5m.
As part of the transfer, Celtic midfielder Ryan Christie will move to Aberdeen on a season-long loan. Hayes, who recently featured for Ireland in the 3-1 friendly win over Uruguay, was the only non-Celtic player nominated for PFA Scotland Player of the Season for 2016-17, a campaign where he scored 11 goals and made 24 assists for the Dons.
Aberdeen, meanwhile, have announced they have “reluctantly” granted permission to Sunderland to speak to manager Derek McInnes.
The Black Cats have been targeting McInnes to replace David Moyes, who left in the wake of their relegation to the Championship last season.
The statement issued by the Scottish Premier League side read: “The club can confirm that early this afternoon Sunderland FC agreed to meet all the contractual obligations for both Derek McInnes and (assistant) Tony Docherty and they have, reluctantly, been granted permission to speak with both of them about the vacant managerial position at Sunderland.
“Aberdeen FC will be making no further comment at this moment in time.”
Meanwhile, Southampton have sacked manager Claude Puel with immediate effect.
The Frenchman took over at St Mary’s last summer and guided Saints to the EFL Cup final, where they lost to Manchester United, and an eighth-placed finish in last season’s Premier League.
But the board has opted to make a change, dismissing the former Lyon and Nice boss.
A statement from Southampton said the club had “terminated the contract of Claude Puel with immediate effect”.
It continued: “Everyone at Southampton would like to express our thanks to Claude for his hard work and commitment this year.
“The highlight of the season was a memorable day at Wembley in the EFL Cup final, a day our fans will always treasure. We wish Claude well for the future.
“The search for a new management team is under way.
“We are confident that we will find the right fit in line with the long term vision of the club.”