I think the reality is the following:
Since Fitz and Toffoli were far apart in contract negotiations, and given that the Devils are most likely not making it to the post season - you load up on assets by trading him now.
Also, can there be a gentleman's agreement between Chevy and Fitzy on Helleybuck in the off-season? Not likely, but this could be a good will trade between the two sides that sets something bigger up for the off-season.
Also, not very likely Toffoli wants to live in Winnipeg, so he might come back to NJ in the off-season anyways.