Hi,
In Standard SAP, Once we create the Invoice , we can't adjusted the Qty in Original Invoice ( Qty Modification is not possible in Invoice Level).
Work Around Solution are:
1. Rather than creating the Original Invoice, create the Proforma Invoice and send the material, Once customer Confirm the Qty then raise the Original Invoice to Customer with confirmed Qty.
Config: In Transaction Code: VOV7, Billing Relevance Should be K.
2. Use POD Concept ( Proof of Delivery). to better understanding the concept check in Google so that you can find No.of documents.