A small tip: I just create a purchase order and register the quantity. Either directly on the purchase order or through the mobile app, advance warehouse in D365 Finance and Operations. Now I want to cancel that line or unregister the quantity for that line.
As compared to Ax 2012, it is relatively easy in D365 Finance and Operations.
For this, open the purchase order and go to a specific line, click on the update line, and click on registration.
From registration form click on ” Add registration line”
On clicking add registration line, you will find negative values.
Click on Confirm Registration. This will reverse the registration process after closing this form you can remove the line from purchase order
Similar way you can unregister the picked quantity in Sales order processing.