I also thought it was just an accounting issue at the beginning, but the accountant told me that there wasn't any extra unclaimed money she received, so that was why she told me she had to bill me on my credit card and later had to interview the 2 male staff on duty in that morning after receiving my complaint. I usually make sure I get a receipt when I pay cash during check out, but that mornng something distracted my attention and my brain didn't function well that early, and I thought the paper I was holding was a receipt. When I think twice, I have a feeling that the staff did it intentionally for not giving me a receipt.

Also after I paid cash, I returned back to my room to get my luggage and went back downstair to hand out my key card to the front desk. He should have given me a receipt the 2nd time I returned the front desk even if I forgot at the beginning.

Besides, if it was simply an accounting issue, then the manager didn't even has to mention "confidentiality laws". He could have mentioned it as an purely accounting error instead.