If 2 puppies cost 5 essence, then by having 3 essence to spend you should really only get 1 and 1/5th of a puppy.
In all seriousness, this is I believe how essence-related quests should work, even if it is a bug. Otherwise limiting quest options because you're a point or two short isn't that great. Maybe if quests had more options in general (like several ways to help) it could be absolute, but when your only options are "Kill" and "Help", shutting out "Help" and forcing people to kill to complete the quest isn't very good.
