If you want to post Invoice only for those line items which you have posted goods receipt, then you should tick the GR bsd IV for all line item both POs. Otherwise, system will pull all line items which hasn't the tick.
Also if you are aware that vendor will deliver the material A1 for PO Z0001 and material B1 for PO Z0002, then you have the option to choose only these line items at the time inbound delivery creation. Why do you need any exit or program ?
Well, at that position, your system will bring all line item for PO Z0001 (because it hasn't GR bsd IV) and the line item (which has GR) for PO Z0002 in your invoice screen and you can remove these line items (which hasn't any GR) by pressing 'Delete unselected items'.