Purchase Inward Tollerance

I am using NAV 2009 + LS Retail, we have raised a purchase order for 1 Item + 1 Variant for 100 Qty, but we have received 104 Qty.

while updating the Warehouse receipt line with 104 Qty, the system has to archive the PO and the quantity should be updated with the excess qty 4. the excess qty should not exceed 5% of the actual quantity 100. the system should allow user to update the
Qty upto 105. if exceeds 105 system has to generate error report that "You are exceeding the Tollerance".

Also the system has to maintain details of excess updated in a seperate Ledger Table.


Thanks.