How to post adjusting entry?

Question

$500 of the recorded sales are payments for goods tht will be delivered in next accounting period.

Answer 1

From what I understand, you've received $500 meant for delivery in future and you've:
Dr Cash 500
Cr Sales 500

Since you haven't delivered the goods, they're not sales, so you've to reverse that. You owe the customers the goods, so you've liabilities to take up. The adjusting entries:
Dr Sales 500
Cr Unearned sales revenue 500 (a liability a/c)