$100 missing how fucked am i

i was a few days ago promoted to key holder (not officially yet though) and my boss called me last night and she said that my till was Short $100. I let another employee use my till because they were only there for 3 hours, so i figured I’d wait until the other people that came in for closing came before swapping tills. I don’t have the money to pay it back, how fucked am i. I have had an otherwise clean record I don’t want to get fired man