When you reclassify the billing offset balances in the first period, the accounting entry is:

Dr Accounts Receivables 10000 - Cr Unbilled Receivables 10000

This entry reduces the unbilled receivables balance and increases the accounts receivables balance by the invoice amount. This is because the invoice has been created and sent to the customer, and the billing offset account is no longer needed. Reference: https://docs.oracle.com/en/cloud/saas/project-portfolio-management/20c/oapfm/manage-project-invoices.html#OAPFM2356091