The I-procurement workflow needs to be configured so that an email notification is generated when an invoice is received for a PO but the goods have not been receipted. In this scenario, the Invoice automatically goes on hold.
The email should go to the Buyer and the Requestor.
There is nothing in iProcurement that will help you do this.
You may have to customize the invoice approval workflow if you use that.
If not, you can write a period alert that fires when such invoices are created. You can also fire custom code in the alert and put the invoice on hold.
BTW, if you do a 3-way match, then the invoice will automatically go on hold since there is no receipt.