User is able to allocate Qty that is equal to current SO Qty. To enter Qty for splitting go to SO edit form -> Actions -> Split Customer -> Split Sale Order pop-up. Enter Allocated Qty that is equal to Qty

User is able to allocate Qty that is equal to current SO Qty. To enter Qty for splitting go to SO edit form -> Actions -> Split Customer -> Split Sale Order pop-up. Enter Allocated Qty that is equal to Qty
