How to compare/calculate between two columns from different tables with missing values?

Hi Joshva ,

bjertf , thanks for your concern about this case. I tried to create a sample data myself based on the user's requirement. Please check if there is anything that can be improved. Here is my solution:

1.Create simple data:

2.In Power Query Editor, append the two tables into a single table:

The relationship is as follows:

3.Create a new measure:

Measure = 
VAR _sum1 = CALCULATE(SUM(A[Amount]),FILTER(ALL(A),'A'[ID] = MAX('Append1'[ID])))
VAR _sum2 = CALCULATE(SUM(B[Amount]),FILTER(ALL(B),'B'[ID] = MAX('Append1'[ID])))
RETURN
_sum1 - _sum2

4.The final result is as follows:

Best Regards,
Zhu
Community Support Team

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If I misunderstand your needs or you still have problems on it, please feel free to let us know. Thanks a lot!