Is there a way to sum items through a portal, but only if the match field meets specific requirements (i.e. a specific text string and its associated number).

In plain language it might look something like:

If the contents of the field in the portal "x" are "Expense", then add those contents and put the result in a field call "Total Expenses".

Any help appreciate. Just can't seem to get my head around a conditional sum within an If statement, which is referring to a portal field

That's because it can't be done that way.

You want to set up a different relationship based TWO fields and sum across that relationship; it's called a filtered portal.

Say your portal is Project to project iine items, so the current portal matches on projectID. Add a global field in Projects, gType. Set up your new relationship to match on Project ID and gType = Type (the field that holds "Expense" ) Enter Expense in gType.