In my business, scenario is like this:
Purchase will create PO, for say 1000 qty. (single line item)
Source inspection is activated in Q-info record.
Now source inspection should be carried our as per the qty available @ vendor site at that time (say 150 qty is ready for inspection)
Now requirement is source inspection lot should be created for 150 qty.only
As per my understanding QI07 will understand the total PO qty. & creates the lot.
So how it is possible to create inspection lot for 150 qty against the total PO of 1000 qty.
You may change the delivery schedule in PO for the available quantity. (For example - 150 PC on 10th, 850 PC on 30th). Then try to create source inspection lot.