Problem with TOPN

I have a working version of the code,

VAR BoundaryAmount =
	CALCULATE( [Amount], ALLSELECTED( 'IDs and others' ) ) * 0.2
VAR CurrentAmount = [Amount]
VAR SummaryTable =
	ADDCOLUMNS(
		SUMMARIZE( ALL( 'Table' ), 'IDs and others'[Index] ),
		"@amount", [Amount]
	)
VAR TopSellers =
	FILTER( SummaryTable, [@amount] >= BoundaryAmount )
VAR AmountTopSellers = SUMX( TopSellers, [@amount] )
VAR TotalAmount = SUMX( SummaryTable, [@amount] )
VAR AmountOthers = TotalAmount - AmountTopSellers
VAR OthersSelected =
	SELECTEDVALUE( 'IDs and others'[Index] ) = "Others"
VAR Result =
	IF(
		ISINSCOPE( 'IDs and others'[Index] ),
		IF(
			OthersSelected,
			AmountOthers,
			IF( CurrentAmount >= BoundaryAmount, CurrentAmount )
		),
		TotalAmount
	)
RETURN
	Result

The only thing which would need tweaking is the ALL( 'Table' ). That currently gets rid of any filters, but you will need to rewrite it to selectively keep the filters you want to apply, while removing the filters coming from the visualisation itself.

One key change is that you do not want to return a value if it is less than the boundary - that should instead be grouped into "others".