Need help with Investment data calculations please!

Make sure you have a date table, marked as a date table, linked to your Posting Date column in a one-to-many single direction relationship.

You can then create measures like

Rolling Amount =
VAR MaxDate =
    MAX ( 'Date'[Date] )
VAR Result =
    CALCULATE (
        SUM ( 'Table'[Amount] ),
        'Date'[Date] <= MaxDate,
        'Table'[Category] = "Investments"
    )
RETURN
    Result

The monthly difference is just the sum of the amount column

Monthly difference =
CALCULATE ( SUM ( 'Table'[Amount] ), 'Table'[Category] = "Investments" )

and the cash movement would be

Cash Movement =
[Monthly Difference]
    - CALCULATE (
        SUM ( 'Table'[Amount] ),
        'Table'[Category] = "Movement on investment"
    )

Hi RichOB 

Try this calc column

Rolling Invesments = 
IF (
    'Table'[Category] = "Investments",
    CALCULATE (
        SUM ( 'Table'[Amount] ),
        FILTER (
            'Table',
            'Table'[Category] = "Investments"
                && 'Table'[Posting_Date] <= EARLIER ( 'Table'[Posting_Date] )
        )
    )
)

But you can actually do this with measures

What I dont get is for September, your cash movement is -435 when -500-65 = -565

Please see the attached sample pbix for the details.