Sheffield United have sold Iliman Ndiaye to Olympique Marseille.
The striker leaves in a 20m deal.
Blades boss Paul Heckingbottom lamented: "I wish Iliman well, my time here has been entwined with Iliman's.
"We could've not sold him and run the risk of him leaving on a free transfer next season.
"Unfortunately for us and me, my job's to try and, while developing players, get the most competitive team on the pitch and in this instance it doesn't go hand in hand."
"It's his boyhood club, his dream club," said the Blades boss. "Hopefully he goes on and has the time of his life similar to what Billy [Sharp] did when he came back to Sheffield United.
"That's the battle we were up against. That's the problem. Iliman turned down vast amounts of money last January to stay with us and help us get promoted so it's not about the money."