OK, I did it!
Hilton Prague 30% (or more - can't remember) less than direct booking.
Worked absolutely fine but no HHonors points and no Exec Floor upgrade although they claimed the floor was fully booked. Actually, I think I did get exec floor ( certainly had lounge access) but no suite which I have had previously as HH Diamond.
When you confirm you get a voucher from an American Tour operator - Tourinco, or something similar.
If the price is good and you can do without the hotel points/benefits then go for it.
PS I think you can't cancel....