Hasta La Motion!

I’m relatively new to Maya, and I stuck on something. I have a CG character that, when eventually composited, will enter my scene from behind a building. So in Maya I have the character walking on a ground plane, and I set up a "physical sun and sky" light and another directional light to simulate the position of the sun. The ground plane has a "Use Background" shader applied and everything looks good. The character casts shadows on the ground nicely. The render looks good. But I also need the building to cast a shadow on the character when it emerges from behind it. So I made a cube and scaled and positioned it to simulate where the building is, and put the"use background" shader on it. Now the cube makes a nice shadow on the character, and the render looks great. Except.. the cube also casts a shadow on the ground, which i don’t need. Is it possible to have the cube only cast shadows on the character? Or do I need to set up seperate lights for that? Any clues?