What happens if you lose a bill at work?

I just got a job and today was my first day not doing training and working the register. My manager told me that we put 20 dollar bills directly into the safe and not in the register so that people dont try to rob the place. It's a 7-11. Today I put a bill down below the counter with the intention of putting it in the safe after I get done serving the customers because thats what he told me to do, and after I did a few sales, I looked down and it was gone. He may have put it in the safe for me because he did that several times that day, but im afraid that I might have lost it or something and I didnt get a chance to ask him. I looked on the ground and didnt see it and Im afraid what will happen if they think that I stole it, I mean its my first day!. What will happen if they accuse me of stealing money? Are there usuallly cameras that can prove that I did not steal it? I dont care if they take the money out of my paycheck I just dont want to lose this job. Am I being paranoid or should I be concerned? Does this kind of thing happen a lot?