Answer: Debit Accounts receivable for $600.
Step-by-step explanation:
The customer had not been billed so that means that they still owe the company. This would make them an accounts receivable so the adjusting entry will have to debit the Accounts Receivable account for $600 to show that it is increasing.
This amount will be credited to the Accrued revenue account to show that the cash has not yet been received.