When you do a cash and points reservation, you are basically buying enough points so that you can make the award reservation. I believe this is why you don't end up paying taxes for hotel reservations (in contrast to SPG cash and point awards, where you do pay taxes on C&P reservations). This is also why you don't get your money back if you cancel a points and cash reservation.

So, the hotel never sees the cash you paid. They just assume you paid for the reservation with points. I don't think it is possible for them to give you a receipt showing you paid cash.