I don't believe so. I think when you try it will revert to the higher prices. Saving an offer on the website does not guarantee a price. Only booking with a deposit or getting an agent on the phone to put a hold on it does that (that is usually done for people that want to mail in their deposit vs. pay by credit card - they will hold it for ?maybe? 7 days).
I'm guessing whatever you looked at the other day is now sold out for the discount for those dates.