Delivery quantity adjustment prior to Goods receipt

Hi,

ASN quantity is 100 PCS in 10 HUs and received quantity is 95 PCS, which makes short of 5PCS.

I have done simple unloading of all 10 HUs. During goods receipt process in /SCWM/GR, one of the HU was short of 5 PCS. Hence  I was entering exception code I001/I002, to reduce the quantity. Here error message comes "Quantity adjustment is not possible due to confirmed quantity".

Is there any configuration needs to be done or any process has to be followed to resolve this,

Karthik