This issue has been fixed in the version 14 release of Sage 300 Construction and Real Estate.

As a workaround until you install that version, you can double click on the blank blue/grey bar at the top of your screen, above the column headings and below the main application toolbar. This will undock the Assign Total toolbar from the window allowing you to view the addition and subtraction options.

You can also use the - key and Shift +, to toggle between subtraction and addition if you have difficulty getting the undocking of the assign totals toolbar workaround to work.

You will need to close the design in order to exit the Assign Totals mode.

Save the design before closing, if you want to keep the total that had just been assigned.