You will have to rotoscope the soles of the shoes. Roto brush may automate the process but most likely you will have to roto by hand. Three seconds, hand Roto, depending upon the movement in the frame – two to four hours.
Once you have created a Matte I would use that as a track matte for a black solid with the blend mode set to multiply. Just the opacity of the black solid until you get the shoes looking the way you want them to look.
If you know nothing about Roto then type "Rotoscope" in the search help field at the top right corner of AE and check out the excellent community resources on the subject.
Here is a really brief look at Roto by hand:
I was hoping there would be a simpler solution but if that's what has to be done then so be it.
Thanks for your quick response.
Before starting the hand rotoscoping (I agree with Rick, no other way to do it) be sure this client of yours isn't going to come back and ask you to do the laces, too. Post production corrections are expensive because they are the result of bad planning and they require delicate execution. Be sure you are billing adequately for this service.
Thanks Bogiesan. It's a short film for a film school grad so I'm working for free however I don't mind as it will build my skills for the future.