According to free dictionary, "Says who?" is used to express disagreement with or defiance against something that someone has just said. And give this example:
A: "There's no way a pipsqueak like you can win this race!"
B: "Oh yeah? Says who?"
I'm wondering if I can replace it with "Who says?". Will the meaning be changed? Is there any difference between these two phrases?
Update with the definition "Who says?" from the dictionary
used to disagree with a statement or an opinion
A: "You can't just take money from the register like that!"
B: "Who says? I own the business, so it's technically my money."