Dynamic Column Difference in Matrix

Hi Anon2020 

It seems to me that you're trying to show the min and max selected years and a column for the difference, if so try this DAX measure

Min Max Difference = 
VAR _minYr =
    MINX ( ALLSELECTED ( CalendarTable ), CalendarTable[Year] )
VAR _maxYr =
    MAXX ( ALLSELECTED ( CalendarTable ), CalendarTable[Year] )
VAR _years = { _minYr, _maxYr }
VAR _minYrValue =
    SUMX (
        FILTER ( ALLSELECTED ( CalendarTable[Year] ), CalendarTable[Year] = _minYr ),
        [Total Sales]
    )
VAR _maxYrValue =
    SUMX (
        FILTER ( ALLSELECTED ( CalendarTable[Year] ), CalendarTable[Year] = _maxYr ),
        [Total Sales]
    )
RETURN
    SWITCH (
        TRUE (),
        NOT ( HASONEVALUE ( CalendarTable[Year] ) ), _maxYrValue - _minYrValue,
        SELECTEDVALUE ( CalendarTable[Year] )
            IN _years && HASONEVALUE ( CalendarTable[Year] ), [Total Sales]
    )

Please see the attached pbix.