"Excuse me; do you have a case quarter?"
A twenty-five cent coin; exactly one quarter.
"Twenty-five cents," or "a quarter," for contrast, could mean any combination of money adding up to $0.25: two dimes and a nickel, for example.
The etymology of this expression is obscure. "Case dollar," "case nickel," "case dime," and "case quarter" originated in South Carolina. The terms may have come from "caser," which has been slang since the mid-19th century for the American dollar or a British crown. "Caser" comes from Yiddish, which comes from the Hebrew kesef, or "silver."
"Case quarter" is a Southern United States term used only in certain social circles. It's often considered African American slang, but there's no evidence to support this claim.
People don't often need a "case quarter," which may be why the term has not spread more widely. "Case quarter" survives because it describes the subject tersely and precisely.
The Oxford English Dictionary, 3rd Ed.