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."
Best Answer
"Who says?" is a completely normal, grammatical sentence, and can be used in that way as well as other ways.
"Says who?" is an idiom, with non-standard grammar:
First, the inversion is not normal. "Says" often shows inversion in narrative, where the quoted utterance precedes (eg "'There he is,' said Jane"), but it is not normal when nothing precedes it.
Secondly, when it is used non-interrogative form (often as a reply eg. "Says my father!") it is usually invariable and doesn't take normal grammatical forms: "Says you!", "Says me!" (rather than "Say you!" "Say I!").
I would say that it is more emphatic, or even agressive, than "Who says?"