option 1: Use the Last function in a calculation field to display the last related record from the portal's table--which is the record your sorted portal lists first as I read your post.
option 2: instead of sorting the portal, sort the relationship in Manage | Database | Relationships. Then you can simply place the field from income directly on the layout. Since your sorted relationship makes the latest expiry date the first record, this is the record that will supply the data to this field.
Option 3: make another portal just one row in size with the same sort order and make the portal borders invisible so that it just looks like another field.
Option 2 was the easiest. Many thanks! Couldn't get Option 1 to work. Option 3 didn't allow me to sort the Table View of the Membership table by expiration date.
"Option 3 didn't allow me to sort the Table View"
As an aside, portals do not function at all in table view; it will be as if the portal does not exist and you placed the related field directly on your layout.