Overview:

Negative Inventory is mostly used for when you want to Sell an item which has a negative amount in your inventory.

For instance, lets say if you have a type of order (work order, In-Store Pickup, Deliver, Ship) and in that order you have that certain item that is out of stock and you recently ordered them from your supplier which means that it will be available soon.

Steps:

To allow negative inventory into your Sales,  you may want to Navigate to Settings>>from General column list Click on Advanced Settings>>Register Settings Tab, finally Allow Negative Inventory option change it from "No" to "Yes".