So, i found solution, for hardcoded name of column:

Sum(iif(Fields!Date.Value= "2017/10", Cint(Fields!Position.Value),0))

Unfortunately, i cant replace string "2017/10" with expression Max(Fields!Date.Value, "DatasetName") because it's inside Sum aggregation,...

I think, problem is that @test-2 table contains 2 rows. So the variable assign is applied twice.
In this case @T1Id, while reading first row of @test-2, the value is...