I often have Purchase Orders that have more then one bill applied to it. I would like to create a custom field that will give me the SUM of the bills/bill credits applied to the Purchase order. Then I would like to subtract that amount from the Total of the Purchase Order. I can not find the right formula field to accomplish this.

I have tried :

{total}-(Sum{appliedtotransaction.amount}) but the only value that is returned is the PO total.

I have created a saved search that returns the bills or bill credits and I have selected the Show Totals checkbox in the results tab and this is the value I would ultimately like to subtract from the Purchase Order total.