That 3.5M was what Parma paid Juve for the half of Seba they currently own. There's no predetermined buy-out.
I saw some quote not too long ago from Seba saying that all he was thinking about was returning to Juve, and he would fit on the left up front, to alternate with Vucinic. It seems as though Juventus will re-purchase his half for around 8-9M. I guess we'll see.