Which internal control(s) would you recommend to prevent the following situations from occurring?

1. While entering the details about a large credit sale, a clerk mistakenly typed in a nonexistent account number. Consequently, the company never received the payment from this customer.

2. A customer filled in a wrong account number on the remittance advice. Consequently, a clerk entered the same number into the system, and the payment was credited to another customer’s account.

3. After processing a large sales transaction, the inventory records showed negative quantities on hand for several items.