Make a copy of the channel you just masked out and begin shading
it to match the light source you've established in your picture.
Apply a Gaussian Blur of a few pixels when you have it shaded
the way you like it.
Copy this channel (select all, COPY) and paste it into it's
own document. (ctrl+N , ctrl+V). Make sure the new document's
mode is set to "Grayscale"
the new document and save it as "displacement.psd"