After reading all your thoughts and talking with my sister in law who works in a bank, I am convinced that this needs to be followed up. You all are right. If at the end of the day he had to look up my account to withdraw $100 and fill out paperwork to do so, then why did he not look at the bank copy of the cashier's check to confirm the correct amount that he "thought" was $1,503 instead of $1,403? This really is intentional theft, whether to cover up a mistake somewhere else or to put it in his pocket!
Thinking of talking to the district manager or someone else higher up the chain! Oh boy, my local newspaper would love this story.