You will probably need pheonix tools fur shaders. You can rent all of their plugins (or almost all) for around $1,600 U.S. per year.
Otherwise you may have to use a picture and rotoscope on top of it. Then matt out where the grass covers the shoes. Just like hair, grass has always been a pain. If you think back on the work that has been done to date, you don't see much grass in anyone's work. It is either sylized, or done by someone like Pixar who can write special shaders.
Urp