I did a pre-paid stay at the Doubletree El Segundo last December. I received a regular receipt at check-out; might be because I ordered room service, but it did show the room amount & the advance 'deposit' or pre-charge. I'd have to pull the receipt from last year's files to say exactly what it said, but I had paper in my hands when checking out.
And I'm sorry - I DO think it is the responsibility of Hilton to provide a regular receipt, whether the room was pre-paid or not. That goes w/ being in the hotel bizness. Or put a disclaimer on their website saying that if you book pre-paid you can't get a receipt. Without that disclaimer, I'd be writing the Prez of Hilton saying send me my receipt or get your programmers to fix the problem w/in the system.
And one time just after 9/11 (it was the Langham Hilton) when I did run into this problem, I wasn't reimbursed by client until I got my credit card statement showing that the Langham had indeed charged my cc. Submitting the confirmation print-out wasn't good enough.
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Sharon