The Germany international was allowed to come off contract at Schalke this summer and is now in advanced talks with Palace.
Heidel said, "It is quite normal that the club contacted us, it is related to the training of the player. Retroactively, we then receive a payment. I think Max is doing the medical check right now.
"I really mean it honestly: I'm happy for Max that he has found a club. He is a top footballer and is now playing in the Premier League.
"He has to prevail."