In the previous versions, it was very difficult to correct a packing slip. you were supposed to reverse the original packing with negative quantity then posting a new packing slip with corrected data.
AX 2012 comes with a good functionality which can really help you to correct or cancel a posted packing slip both in sales and purchases.
After posting the purchase packing slip
go to the Receipt and click on Product Receipt, where you can see the list of packing slips posted. select the packing slip which you want to correct and click on correct.
The same form that is used for posting the original packing slip will get opened, now do the necessary changes to that packing slip(example: quantity) and post the packing slip. Now you can observe the changes to the posted packing slip.
