Pay Rate Is Lower Than Signed Job Offer Letter. (missouri)

I recently started a new job that originally stated they would pay me 17 dollars an hour. After getting a verbal offer for the job, I received a job offer letter that had offered a rate of 19 dollars an hour. I chalked up the pay increase to the fact that I have a double major and serval certifications when this job doesn't require you to have a degree or certs. I signed the offer letter about two weeks before my first day which I had downloaded after signing it to have a copy of it featuring the date and time of when I signed it. I got my paystub for my first pay check today (payday is Friday) and noticed that it had a rate of 17 dollars an hour and not the 19 that was in the offer letter. My supervisor is escalating it to see if they'll honor it but said he's not going to promise it. I just don't know how this can't be legally binding, but then again I live in Missouri where you can get fired for any reason. So I just want to know do they legally have to honor the 19 dollar an hour offer or can they just say it was an accident and I'm still only going to get 17/hour?