How to get the difference between more rows?

KimBoehmer 
If you want to follow my suggestion and disply the results in a tablevisual, then place the Sorce Name column in the table along with the following measure. From the visual format options, activate the total and rename it "Difference"

Value =
VAR MeasureValue =
    SUM ( 'Zusammenfassung'[Value] )
VAR T1 =
    ADDCOLUMNS (
        VALUES ( 'Zusammenfassung'[Source.Name] ),
        "@Value", CALCULATE ( SUM ( 'Zusammenfassung'[Value] ) )
    )
VAR DifferenceValue =
    MAXX ( T1, [@Value] ) - MINX ( T1, [@Value] )
RETURN
    IF (
        HASONEVALUE ( 'Zusammenfassung'[Source.Name] ),
        MeasureValue,
        DifferenceValue
    )

If you wish to display the difference in a seperate chart, then place the following measure in the chart ALONE (don't place any column along with it in the same chart)

Value difference =
VAR T1 =
    ADDCOLUMNS (
        VALUES ( 'Zusammenfassung'[Source.Name] ),
        "@Value", CALCULATE ( SUM ( 'Zusammenfassung'[Value] ) )
    )
RETURN
    MAXX ( T1, [@Value] ) - MINX ( T1, [@Value] )

Hi KimBoehmer 
Please refer to attached sample file with the solution. I've created a new column using power query to get the Source Number for sorting purposes.

Value = 
VAR MeasureValue =
    SUM ( 'Zusammenfassung'[Wert] )
VAR T1 =
    ADDCOLUMNS (
        VALUES ( 'Zusammenfassung'[Source.Name] ),
        "@Date", CALCULATE ( MAX ( 'Zusammenfassung'[Source.Number] ) ),
        "@Value", CALCULATE ( SUM ( 'Zusammenfassung'[Wert] ) )
    )
VAR FirstRecod =
    TOPN ( 1, T1, [@Date], ASC )
VAR LastRecod =
    TOPN ( 1, T1, [@Date] )
VAR DifferenceValue =
    MAXX ( LastRecod, [@Value] ) - MAXX ( FirstRecod, [@Value] )
RETURN
    IF (
        HASONEVALUE ( 'Zusammenfassung'[Source.Name] ),
        MeasureValue,
        DifferenceValue
    )