Refers to coins or small-denomination money, often used when breaking a larger bill or needing exact change.
Do you have any small change?
I'd like to break a 10,000 yen bill into small change.
大金 means a large sum of money, the opposite of 細かいお金 which refers to coins or small-denomination cash.
お釣り specifically means the change handed back after a purchase, while 細かいお金 just means coins or small bills in general, not necessarily change from a transaction.
A common phrase combining 細かい (small, fine) and お金 (money). The prefix お adds politeness.