The earliest entry the OED Science Fiction Citations Project has verified firsthand is from 1937, the serialized publication of E.E. Smith's "Galactic Patrol" (novel 1950).

John W. Campbell himself used the term in his 1947 novel The Mightiest Machine--but that was published as a serial starting in 1934, so that might actually represent the earliest use of "credit" as currency.

Edward Bellamy's Looking Backward (1887) uses credits.

In Bellamy's future, trade has been done away with as the nation is the only producer of goods. Consequently there is no need for money so a system of credits replaces it. Distribution of goods is handled "on the simplest possible plan... A credit corresponding to his share of the annual product of the nation is given to every citizen on the public books... and a credit card issued him with which he procures at the public storehouses... whatever he desires, whenever he desires."

Credits aren't exactly used as a unit of currency here but there's certainly a fairly close similarity.

The book is more a soft sci-fi though. It's set in the future and does deal with new technology and scientific advancement but it is at its heart a socialist utopian novel and that's shown pretty clearly in its emphases.

Don't know how useful you'll find this as I suspect it wasn't entirely what you had in mind with the question, but I think it's quite interesting anyway.