Ignoring a relationship

Dear experts, 

In the following data model

I am trying to calculate some costs in G_L that do not have a Case ID. I need to ignore the relationship between G_L and Cases. 

The G_L table is the following:

As you can see, when I select a year, the rows that don't have a Case ID are logically ignored:

The measure that I'm trying is the following:

Measure = 
CALCULATE (
    SUM(G_L[Amount]);
    FILTER ( G_L; G_L[DEPT] = "610" );
    FILTER ( G_L; G_L[PROJECT] = "FO" );
    FILTER ( G_L; G_L[G_L Account Nr] = "3110" || G_L[G_L Account Nr] = "3130" );
    USERELATIONSHIP ( G_L[Posting date]; 'Date'[Date] )
)

However, as you can see in the 2 screenshots above, I'm getting blank results. 

The desired result is 3600.

I have uploaded this dummy file here: https://drive.google.com/file/d/1XEmbc4Z0DOq_cG3OU3ZaVi4FL8fc97qJ/view?usp=sharing

I would really appreciate some assistance 🙂