Having Friends

So I work retail, there was two other times I was short for my till but my manager told me not to worry about it. Recently, someone tried to add money to their card and we do that! It's called financial services, and he wanted 50 dollars on there. The machine glitched out or smth, and told us to refund back 50, now here's the thing, the man gave us a hundred, and during the time the assistant manager was giving me change for my till.

She saw the refund and gave me 50 dollars to hand back to him and 50 dollars to put in my till, that was the mistake. He never actually paid for the card, so he didn't need any change back, but the machine said to refund him and that's what we did, making me.. Short 50. I explained this situation to my manager, and she told me it was fine and how since we know you didn't take the money, you'll get a warning and I was fine with that.

Come into work today, my co worker that literally has the same position as me told me to come to the back. She said me being 50 dollars short was inexcusable and how I was terminated, mind you.. SHE WORKS THE SAME POSITION AS ME. So I was confused on why not the assistant manager nor the manager but SOME WOMAN THAT HAS THE SAME JOB AS ME, was firing me. But this is my first job, I was sad, scared and confused, so I listened. Only until I called my dad when I realized what was wrong. So I'm curious, is she allowed to do that? Should I be calling cooperate??