Hello Arindam,
I assume you would use the alias names for the minus calculation. Alias names cannot be used in that case. Normally in such situations the "expression" used to calculate a specific column has to be repeated in such situations, but the sum function is not supported for minus operations. So within one view you cannot reach your goal.
You have to create a second CDS view on top of your view which adds the minus calculation. There of course you can use the names GOODS_RECPT and GOODS_ISSUE.
For your existing view I would recommend you to add an else value to your case expressions to avoid null values.
Best Regards,
Florian