Of course they should honor the discount. The clerk might have made a mistake, but it happened and you purchased food at the restaurant. Perhaps the only reason you went to the restaurant there, or ordered room service was because you thought you were receiving the discount. I know there have been times the only reason I had dinner at a hotel was because there was a discount, or other promotion.

If it was an honest mistake on the clerk's part I'm sure he will be much more careful in the future when he tells future guests about the discount.

They should have fixed the problem immediately at check out, and this should not have required a discussion with the manager, unless you were trying to get a 50% discount on the wedding reception you had booked prior to checkin to your discounted room.

My thinking is if they are offering steep discounts, promise and recend a discount they are looking for any way to "enhance revenue." I hope it was just an honest mistake, noth that they thought you would either forget or not notice the missing discount at checkout.