4. On September 12, a company provides services on account to a customer for $4,000. However, on September 15, the customer is not completely satisfied with the service and the company grants an allowance on the amount owed of $500. On September 21, the customer makes full payment of the balance owed, excluding the allowance. Record (a) the services provided on September 12 (b) the sales allowance on September 15, AND (c)the cash collection on September 21