What about adding one more field to the data source - called Subtotal (or something that makes sense to you). It doesn't have to show on the form - just add it to the data source. It's default value could be the formula you have above - "Total Expenses" - "Less Company Paid" - "Less Travel Advance" - "Less Personal Expense".
Then, put a rule on this field - that when it changes, if the value is greater than zero, set your Due To Employee field to its value, otherwise, set the Due To Employee to 0.
I'm attaching a simple sample form - save it locally, and right click and select Design to open in design mode. Then you can take a look and try it out in preview.