GR/IR connects the inventory receipt with the supplier invoice. For a matched stock item, the receipt credits GR/IR at the PO value and the invoice debits it. A remaining credit can represent received goods that have not yet been invoiced. It needs an explanation, not an automatic expense posting.
Analyze the document references before netting unrelated items together. The training client's opening PO has received 20 housings and invoiced 10, which leaves a 400 USD GR/IR credit. A new fully matched order can clear its own effect while that opening balance remains. These exercises make that distinction explicit. Use the signed G/L balance and the purchasing quantities to explain why a zero movement on one order doesn't mean the entire clearing account is zero.