As a quick workaround, you can disable the inventory check in void_transaction.php.  The check begins

if ($_POST['filterType'] == ST_SUPPRECEIVE) {

The inventory will go negative after voiding the delivery, But after receiving the PO again, it will go positive.  I haven't tried this, so be careful, and test this first in a demo database.

Another workaround without changing code would be to temporarily make an inventory adjustment, and then delete it afterwards.  The error message that you describe should tell you the stock_id and description of the affected item.

Yes I know this is a pain, and the real solution is for someone to post the code for a true supplier invoice editor that would not apply the inventory checks until the update key is pressed.

The void cannot be processed because there is an insufficient quantity for item.

    1. First void the invoice (Setup->Void A Transaction)
    2. Void the delivery.  Note that the delivery cannot be voided until the invoice is voided.
    3. Purchases->Outstanding purchase orders maintenance: Edit the PO.  You can change date, add or remove items.
    4. Purchases->Outstanding purchase orders maintenance: Receive the PO
    5. Purchases→Supplier Invoices: Invoice the PO

or

Make sales orders instead of direct invoices and then let the staff edit them and proceed to convert them into delivery orders and invoices thereafter.