The 25-year-old expires at the end of December but the Union have an option to extend by another year.
Philadelphia media outlet Brotherly Game relayed what Curtin said about the Leeds links, stating: "Curtin says he's talked with Jesse Marsch about Kai Wagner's status.
"Says that the team would like Wagner to stay, but that Kai would also like to play at the highest level.
"The club are still waiting for official offers to come in. (It is a) still to be determined, kind of situation."