If Z is negative, it's certainly not worth taking the deal. My point was it wasn't worth JPM taking the deal for positive Zs (up to some value). If Z is zero or negative, it obviously isn't worth taking the deal.

That is, not only is Y=X a bad deal, but Y=X+Z is a bad deal for a good deal of positive Zs. Up to Zmax which is a positive value. For all values of Z < Zmax (including negative Zs) it's bad for JPM