This page describes on how to do the General settings
Under the general settings, you can Add discount field for the line items and set the warning on negative stock.
To go to Sales Invoice setup, click on Settings-> Sales Invoice Setup as shown below
The following screen appears when you click on sales invoice setup, click on General as shown below
1. To add discount field for the line item, check the "Add discount field for line items" check box
2. To set a warning on negative stock, check the "Warn on negative stock" check box as shown below and click on save