Mapping different depending on total sum

lekkerbek,

This approach uses two mapping code columns in DimGLMapping:

Data model:

The inactive relationship is between DimGLMapping[Credit Mapping Code] and DimGLClassificationReturns[Mapping Code].

Measures:

Total Amount = SUM ( Transactions[Amount] )

GL Mapping = 
VAR vDebitMapCode =
    MAX ( DimGLMapping[Debit Mapping Code] )
VAR vCreditMapCode =
    MAX ( DimGLMapping[Credit Mapping Code] )
VAR vDebitMapping =
    CALCULATE (
        MAX ( DimGLClassificationReturns[Fin Stmt Line] ),
        DimGLClassificationReturns[Mapping Code] = vDebitMapCode
    )
VAR vCreditMapping =
    CALCULATE (
        MAX ( DimGLClassificationReturns[Fin Stmt Line] ),
        DimGLClassificationReturns[Mapping Code] = vCreditMapCode,
        USERELATIONSHIP ( DimGLClassificationReturns[Mapping Code], DimGLMapping[Credit Mapping Code] )
    )
VAR vResult =
    IF ( [Total Amount] > 0, vDebitMapping, vCreditMapping )
RETURN
    vResult

Note the difference in GL Mapping when the amount is positive vs. negative:

Let me know if this works in your data model. If not, please post a pic of your data model, desired result, and sample data (in a format that can be copied into Power BI).