That sucks....if he has a bag or anything...like a backpack...

You have to follow him around and wait for the oppurtunity....

Just steal it.....and fucking RUN.....and see what you can find...

Maybe disguise yourself with a hood...and dress differently....

MAYBE You'll get more than your 20$ back...

If you don't get any money (or sell anything)....just ruin his shit...and that will be enough revenge...

Just an idea...prolly the best idea you can find in this situation...gotta be sneaky though..