If you have added Custom Fields to the Sale Item object, you may have noticed that you are unable to add your custom field to the Sale Items page. This is because the Sale Item Edit page is a hard-coded Visualforce page. To change the Edit page from Visualforce to the Salesforce "Standard page" follow the instructions below:
Navigate to Setup
Use the QuickFind to go to the Objects menu
Choose the Sale Item object
Scroll to the Buttons, Links, and Actions section
Click Edit next to the page Label for Edit view
In the Salesforce Classic Override section, choose Standard page
Click Save