The Magpies have been tracking the Mali international throughout the summer and are believed to have offered €8million (£7m) to secure his signature.
Sochaux chairman Alexander Lacombe: "There has been an offer from them (Newcastle).
"But my goal is to maintain a competitive team.
"I won't sell Modibo without having found a replacement.
"We hope to finalise the deal for Gradel.
"It's a deal which is at an advanced stage."