Sum of Sales - Refunds in pivot table (both positive values), How to?

I'm new to pivot tables but have manage to get it working almost as i want it but have ran wall right now. It feels like it should be very easy to fix but i just don't know how to do it.

The total value should be Sales minus the Refunds but right now it adds Sales + Refunds, giving a wrong total value. The refund value i take is a positive value.

Is there some easy way to make this work? It should show 8997 as total.

Added file further down.