Invoice Workflow with activation condition


How can I setup a workflow to make sure that only when there is a match variance ( a warning icon is displayed has price or quantity matching discrepancies) that the workflow should be enabled and submitted to the assigned user in the hierarchy???

I tried by creating a WF with the field activation like this : " vendor invoices.last match variance is not OK" but it does not work. The WF is still enable no matter what. In fact , as soon as I create an invoice from a PO or from the Pending vendor invoice screen , the WF is enable for submission and this is not what I want.

I would like to have that WF enable only when I create a discrepancy with the PO price and the invoice price. I am using the Three way matching.