Hello, @shioban.

I can share some information about deleting an invoice in QuickBooks Desktop. 

Yes, once you've deleted an invoice, inventory items that you've added to the invoice will automatically go back to your inventory. 

To delete the invoice, open the invoice that you'd like to delete. Then, click on the Delete icon from the Main tab of your Create Invoices window. 

For more information about creating an invoice from a sales order and estimate, refer to this article: Create an invoice in QuickBooks Desktop.

Once you're done, you can go to the Inventory menu and then click on the Item List to check your inventory items. See this article for more information: Add, edit, and delete items.

You may also consider turning on Advanced Inventory if you need to track inventory stored at different sites or locations. Check this article for more information: Set up Advanced Inventory.

I'm always here if you still have questions about your invoices and inventory. Let me know by leaving a reply below. Have a great rest of the day!