So. now that this thread is basically dead, here's full disclosure. Before I posted this, I called the deli and offered to make a wireless payment or just order something through seamless that they didn't have to deliver. The guy I spoke to acted confused and then put a lady with an asian accent on the phone. She started asking me somewhat detailed questions about the transaction and it started to dawn on me that she wasn't trying to work out a payment, she was determining whether the cashier was at fault. I started to clam up and she just thanked me and told me not to worry about the money.
After the phone call ended, I went out to this communal work area we have in my office. There were about 6 people working in there. I told them that I think I just got somebody fired. I relayed the story a lot like the first post in this thread and ALL of them said that in this situation, it probably would have been better (for the cashier)to just forget about it. In NYC at least, she wouldn't have had to repay the money and most stores with high volume have a reasonable tolerance for unbalanced registers. Everyone seemed to agree that the fact that I spoke to what may have been a manager and basically verified that she made a rather stupid mistake probably sealed her fate.
I felt horrible about what might have happened, but I really don't think anyone would have come to that conclusion if they didn't know how the story turned out. I don't think my co-workers were trying to make me feel bad, but I don't think most people would think there was a possibility the cashier would be in more trouble if you returned the money. And no one in this thread suggested that was a possibility and the voting far and away says that the money should have been returned. I don't necessarily feel better about it, but I was wondering if it was unreasonable on my part not to see that coming.