Barnsley have accused Daniel Stendel of speaking to a rival club before he was removed from his post as manager as they vowed to seek damages from Hearts over his appointment at Tynecastle.
Hearts announced on Saturday that Stendel had signed a two-and-a-half-year contract to be their manager after protracted negotiations.
Barnsley co-chairman Paul Conway had earlier demanded compensation after claiming Stendel was still under contract at Oakwell, despite the club announcing a “separation” on October 8.
And the Sky Bet Championship club have vowed to step up their case, less than 24 hours before Stendel is due to begin work at Tynecastle.
A club statement read: “Early in the season, Daniel Stendel entered into negotiations with a rival Sky Bet Championship club to take over their managerial post and recruited other members of contracted Barnsley Football Club staff to follow him.
“Never once did Mr Stendel seek approval or alert members of the club’s board to these discussions. Only when approached, by the board, was there any sign of acknowledgement.
“This was a clear violation of Mr Stendel’s contract and created additional unnecessary distractions for our club which was recently promoted to the Sky Bet Championship.
“This event, coupled with numerous other issues, led to removal of Mr Stendel as head coach.
“Out of respect for his contributions to the tremendous 2018/2019 season, we wanted to settle this matter privately and amicably out of the eye of the public.
“After two months of unproductive discussions (and the subsequent public announcement by Heart of Midlothian that he has joined them as their new manager), the club cannot wait any longer in speaking to its fans.
“More recently we have attempted negotiations of a settlement with Heart of Midlothian Football Club for the services of Mr Stendel, Chris Stern and Dale Tonge to no avail.
“The club will continue to vigorously defend its rights against all parties involved in tampering of club staff so the club recovers all damages incurred. All recovered damages will be reinvested in our squad.”
Hearts are due to unveil Stendel as their new manager on Tuesday afternoon.