Both players had agreed to join the Tykes before the end of the season, with Kell completing his transfer to Oakwell despite breaking his leg in the penultimate game of the season.
Hayes was offered a new deal to stay at Glanford Park, but his form collapsed towards the end of the season as it became clear he was preparing for a better pay deal elsewhere.
Barnsley arrive in North Lincolnshire on August 9 for the second League fixture of the campaign, with Hayes sure to get a hostile reception.
And with the Football League set to look into the transfers, there could yet be further twists.