Mapping different depending on total sum

lekkerbek,

Try these measures. Delete the relationships between DimGLClassificationMappings and DimGLAccountClassificationRelations. 

Total Amount = SUM ( Transactions[Amount] )

Dynamic Amount = 
VAR vMapping =
    MAX ( DimGLAccountClassificationRelations[CODE_ATTR] )
VAR vAccountBalance =
    ADDCOLUMNS (
        SUMMARIZE (
            DimGLClassificationMappings,
            DimGLClassificationMappings[GL Account],
            DimGLClassificationMappings[Debit Mapping],
            DimGLClassificationMappings[Credit Mapping]
        ),
        "tmpTotalAmount", [Total Amount]
    )
VAR vAccountMapping =
    ADDCOLUMNS (
        vAccountBalance,
        "tmpMappingToUse",
            IF (
                [tmpTotalAmount] > 0,
                DimGLClassificationMappings[Debit Mapping],
                DimGLClassificationMappings[Credit Mapping]
            )
    )
VAR vAccountToSum =
    FILTER ( vAccountMapping, [tmpMappingToUse] = vMapping )
VAR vResult =
    SUMX ( vAccountToSum, [tmpTotalAmount] )
RETURN
    vResult

Create visual using DimGLAccountClassificationRelations[DESCRIPTION] (and additional columns from this table).