Can my employer deduct money from my pay with no proof it was taken?

I work for a convience store. I was recently hired 8 days ago. On my 5th day the register came up short. My manager has not igned me a number to run under for my own drawer. I was also asked to come in early that day so she could leave to finish packing for her move (same city). When I arrived she told me I could run the register under her number. She never closed her register out and did a check out. The register was open under her number for 17hrs. During the night we had a man that wanted to do a lotto pay out of $290.00. My co-worker asked me if I had that much in my register and I told her I would check. I hit lotto pay out to open the drawer and realized I did not have the money. She then went to her drawer and hit lotto pay out and was able to pay the man. The next day the manager said that I would have to pay the shortage. *IF I didn't give him the money.... it should have been over!" She said $150.00 would be taken from each of our paychecks. I didn't understand how and talked it over with my co-worker later that night. She too was upset. She asked to see the paperwork. The boss showed her my check out sheet. Only I don't believe it was mine. She was shown one sheet with the total scratched out. When I did the paperwork it was on two sheet b/c of having to make so many money drops in the safe. Second, I didn't scratch anything out. Then she was told they have me on camera handing the man money and then her handing him money. I asked to see the tapes. The manager told me the general manager said that was not allowed. I asked whom to speak with to see the tape and she said the GM. About 30mins later she takes me in her office and shows me some video footage of the night. Four screens with none of them directly over the register. I didn't hand the man any money. Now the story is that I gave the co-worker the cash and she set it to the side and they don't know what happned next. I didn't see any of that either. But because they can't prove anything, we are at fault??? Talked with another co-worker and who's mom manages another store and it was revealed to me that she could have enlarged the screen and showed more of the register and what happened. I signed a peice of paper saying I would cover any shortages made in my register. That was not my register.... it was my boss's. Plus it was open for 17hrs under her number with no cash out. What steps should I take next? All the manager's are friends and if I call one they will call each other. How should I handle this because I feel if I'm not at fault why should I pay?