Using SELECTEDVALUE function when more than one value is selected in Filter

Well, what is the point of having a model which cannot do a simple thing? There is always an argument, to improve a model to simplify DAX or write complex DAX to achieve the results, and seems you are falling into 2nd category, which is usually not a good idea because it can have a performance impact and a solution which cannot be scaled over time.

Having said that, you can try this DAX function.

Sales_Amount =

CALCULATE(
Sum('Sales_Table'[Sales_Amount]),
REMOVEFILTERS('Commision_Table'),
TREATAS ( 
   VALUES('Commision_Table'[Commision Year]),
   'Order_Table'[Order Year]
)
)

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