I would think cash would be local to the star system and depend on their TL, degree of outside contact, and local productivity.

Credits would be interstellar currency regulated by the Empire.

My take on Credits was to use debit cards with an internal microcomputer. Essentially, buyer and seller set their cards to credit and debit the agreed upon amount, verified by DNA that they are who they say they are, (fingertip scan to verify authorized person who is alive and not under duress.) The users tap cards or use a credit interface over coms and the transaction is complete. No Imperial coinage needed.