Error occurred when posting AR credit note for price variance

I should credit to a customer a price variance. As far as I haver read from some books it should be done as follows:
· to enter a new sales order containing a line with a negative quantity and the old price and a second line with a positive quantity and the right price;
· then I apply (at least I try it …) the Inventory/ Marking to connect the two transactions with the original line that is already invoiced to offset the inventory value;
· then I try to post the packing slip for this sales order with the negative and positive line in order to be able after that to post the credit note as well ( I am not sure if I need to do this step) but the following a message appears that tells:
A quantity cannot be picked.
Insufficient inventory transactions with status Picked. Operation canceled
Could anyone tell tell what exactly I have missed to do ???
thank you in advance!
Best regards,