Spoiler (click to show/hide):
Although I wrote it in AS3 the main code for doing the actual skewing should be basically the same:
Spoiler (click to show/hide):
the matrix is just a copy of the transform matrix from the box.
[Edit]
I just realized that that code is technically wrong. instead of checking if varix and variy are both positive you should be checking box.x-275 and box.y-200. in my case it does basically the same thing because my box is more or less exactly in the center, but if you want your box off to one side, you need to check its x and y position instead of the mouse's.