This is the same situation as Facebook's IPO with NASDAQ. Orders were placed $40, then cancelled, then filled at $40 six hours after the market closed (at which point Facebook was already trading below its IPO price).

Unless you are a massive financial institution with some serious clout/legal department, I recommend you cut your losses and move on.

Your time is much more valuable spent doing something else rather than the time you will spend trying to recover $X,XXX