In a scenario where Cash sale is created and and billed without creating Item fulfillment
Item fulfillment is not creating and the there is a Fully cash refund, it will increment the total quantity of the item.
In a scenario where Available Quantity is 100
Created a SO/Cash sale with Quantity 10
Quantity available – 90
On hand – 100
Committed – 10
Inventory Status after SO created

Cash refund (Fully-10qty)
To avoid inventory increment, Quantity for Cash Refund alloted ‘0’

If the Quantity in the Sales Order, is given as 0 (10-10) it will results a negative quantity.
To avoid this negative quantity, do not update or change the quantity and check the Closed box in that Sales Order Line

Inventory status after closed the Sales order

Quantity available and Quantity on Hand changed from 90 to 100
Negative Quantity issue is resolved