Help on DAX to compute differences for multiple variables

Hi,

I am not sure how your semantic model looks like, but I tried to create a sample pbix file like below.

Please check the below picture and the attached pbix file.

INDEX function (DAX) - DAX | Microsoft Learn

expected result measure: = 
VAR _diff =
    CALCULATE (
        SUM ( forecast[Value] ),
        INDEX (
            1,
            ALLSELECTED ( forecast[Version], forecast[Version sort] ),
            ORDERBY ( forecast[Version sort], ASC )
        )
    )
        - CALCULATE (
            SUM ( forecast[Value] ),
            INDEX (
                1,
                ALLSELECTED ( forecast[Version], forecast[Version sort] ),
                ORDERBY ( forecast[Version sort], DESC )
            )
        )
VAR _result =
    CALCULATE (
        SUM ( forecast[Value] ),
        KEEPFILTERS ( forecast[Version] IN VALUES ( 'version'[Version] ) )
    )
RETURN
    IF (
        HASONEVALUE ( 'version'[Version] ),
        SWITCH (
            TRUE (),
            COUNTROWS ( VALUES ( forecast[Version] ) ) = 2, IF ( SELECTEDVALUE ( 'version'[Version] ) = "Difference", _diff, _result )
        )
    )