Unpick an item after the status is under packing slip

Hi,

I have a scenario where there is an item which was suppose to be unpicked but the picker picked it. The status is already changed to packing slip. How can I unpick the item .

I have followed the following steps but still I am not able to unpick the item.

I clicked on packing slip journal for the particular sales order I want to work on and cancel the SO. The status changes to "canceled" and document status remains "packing slip".

Then I click on picking list journal and click picking list registration and under the lines when I click on functions > unpick I get an error message saying "Quantity XXX cannot be unpicked because inventory transactions with status Picked are insufficient".

Please let me know how can I unpick an item after it is under packing slip status?