Z axis

[PTR]

I am ordering my game objects on depending on their realY (Y + custom offset), but I need a new axis because I want some objects to "bounce", have physics on that axis. I am currently using the offset to create that, but I need to keep the offsets on X,Y,0. The problem is that when I change the offset to move the graphics up, the collision changes of course. Any idea how could I move only the image itself and not the offset?