Setup:

• Unleashed on Browser

Answer: 

You cannot delete a sales order with a completed invoice. The process for this is to complete a stock adjustment for the products on that order to add SOH and then complete the SO. 

Once the SO is completed, you can process a Credit Note and make sure the return box is ticked to bring QTY sold back into the system. 

Then you will process another stock adjustment to remove the returned QTY out of the system.