Is a printed job offer a binding contract? Is it illegal to change the offer with the first check?

I just started a new job. Over the phone they made me an offer of $36,000 a year and I accepted it. On my second day I was handed an offer letter which laid out in detail the additional expectations for me which include being there a minimum of 50 hours a week for my salary and also a requirement that I remain available and am on call 24/7 for my salary (which was never discussed with me previously). This letter stated that I would be paid $46,000 a year for my duties and broke it down into bi-weekly increments making it clear this was not a simple typo. I let my in store manager know so thst we could correct this on the paperwork I filled out on my first day. I just received my first paycheck which was short of what I was expecting and this check accompanied a second offer letter which details the $36,000 offer instead but still contains the time and on-call expectations. Is there anything I can do legally to get back to the $46,000 offer made to me just 7 days before? Or am I just being introduced to the real spirit of a crappy company early on and am now in need of a new job again. At 36K this was something I could make work while I looked for something better. At 46K I could think of it as a more long term position. At 36K I could work 50 hours a week.....but not more really, as I would need to get an additional job to make ends meet. I know I deserve the 46K and have substantial related work abd education experience which makes that offer much more reasonable than 36K. I also could not allow myself to be on call 24/7 for 36K. Is this retraction of my original offer an underhanded way of firing me?
Any advice is much appreciated. Thank you and have a nice day,
Eve Isaacks