Negative Vendor Invoice line item

When processing an AP Invoice with Receipt Lines (received against a PO), and the extended value of the Receipt Lines is slightly more than the vendor invoice (because the cost on the PO was slightly too high) then you will have a positive variance (meaning you need to reduce the cost of the line item total). If you try to add a Misc Line with a negative amount, it won’t allow it. It appears that you are forced to adjust the unit cost downward on one or more Receipt line items in order to eliminate the variance, but sometimes the variance is so small (perhaps due to rounding difference between our system and the vendor’s system), that you really just want a single line on your invoice to plug the tiny difference. Is there a way to enter a negative line on a vendor invoice?