When the AP department receives the invoice, it records a $500 debit (liability) in the accounts payable field and a $500 credit to office supply expense (received items).
Most contracts are net 30 or 60 days out, this first transaction accounts for that.

The company then writes a check to pay the bill, so the accountant enters a $500 debit to the checking account and enters a credit for $500 in the accounts payable column.
Actual payment. debit on checking because you spent money.
Credit on AC payable because you reduced your Supply Expense (previously credited) from your previous transaction.