"Is it possible for a portal to show the found set when the Salary table has a find performed on it?"
No, but it is possible to set up a relation ship that "filters" the portal to produce the same effect.
On the other hand, a summary report based on the same table as your portal is likely to be more flexible and you can perform a find on it to restrict data to a specific group of records.
Here's a link to a simple tutorial on setting up summary reports that you may find useful:
Creating Filemaker Pro summary reports--Tutorial