Some choices, which one is best for you will depend on your setup.
First though, Country in (@Country) is misleading as @Country is one value, not a list by the time SQL gets to see it, so changing your WHERE clause to Country = @Country would achieve the same result but make it clear you understand that @Country is a single value...
1. add OR @Country='BOTH' to the query (and use "BOTH" as the value for the "BOTH" label in your dropdown list...
2. change to using Country = ISNULL(NULLIF(@Country,'BOTH'),Country)
3. change your query to use an IF
IF @Country = 'BOTH'
SELECT col1 ,col2, col3 FROM tblInvoice
ELSE
SELECT col1 ,col2, col3 FROM tblInvoice WHERE Country = @Country
4. Filter the data in the report and set the value of the label "BOTH" to USA,EUROPE in the dropdown
I am sure there are other ways also, these are just a few.
I would personally prefer options 3 or 4 but it all depends on your exact situation.
MM
select geometry::STGeomFromWKB(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