Continuity mistake: When Cratchett gets the bird from the shop, it's clear that the bird is a duck. But throughout the entire film he's referred to as a goose.
Continuity mistake: When Isabel is breaking off the engagement with Scrooge (in Christmas past), the eye color of Scrooge changes to brown and then back to blue.
Answer: Most likely because he's buying the shop's entire inventory. That would be shocking enough to explain his reaction.