Match Debit Memo to Invoice - unapproved

I want to control the use of Debit Memos matched to POs, but I find this example scenario:

1. Enter PO for 10 items

2. Receipt only some (eg. 8)

3. Enter Invoice for all 10 and validate.  This produces a Qty Rec Hold and the invoice is not validated (as expected)

4. Enter Debit Memo for unreceipted quantity (eg. -2) - and don't approve

5. Because the Debit Memo allows for quantity received variance, the Invoice can be validated (and once approved, is available for payment)

The Debit Memo has not been approved, but it allows the invoice to be validated and paid. 

How do I prevent the invoice to be released for payment until the Debit Memo has been approved?