Using M Query for an 'if else' argument

Hi Anonymous 

You can download my file and click on the steps on the right step pane to see what is applied step by step.

Code in Advanced editor

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLUN9Q3MjC0BDKNzA0MlGJ1YOJGOMSNcYiboIrHAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [uniquekey = _t, #"invoice period" = _t, volume = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"uniquekey", Int64.Type}, {"invoice period", type date}, {"volume", Int64.Type}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 1, 1),
    #"Merged Queries" = Table.NestedJoin(#"Added Index", {"uniquekey"}, rate, {"uniquekey"}, "rate", JoinKind.LeftOuter),
    #"Expanded rate" = Table.ExpandTableColumn(#"Merged Queries", "rate", {"uniquekey", "rateperiod", "rate"}, {"rate.uniquekey", "rate.rateperiod", "rate.rate"}),
    #"Added Conditional Column" = Table.AddColumn(#"Expanded rate", "condition1", each if [invoice period] >= [rate.rateperiod] then [rate.rate] else null),
    #"Filtered Rows" = Table.SelectRows(#"Added Conditional Column", each [condition1] <> null and [condition1] <> ""),
    #"Grouped Rows" = Table.Group(#"Filtered Rows", {"Index"}, {{"Count", each _, type table [uniquekey=number, invoice period=date, volume=number, Index=number, rate.uniquekey=number, rate.rateperiod=date, rate.rate=number, condition1=number]}}),
    #"Aggregated Count" = Table.AggregateTableColumn(#"Grouped Rows", "Count", {{"rate.rateperiod", List.Max, "Max of Count.rate.rateperiod"}}),
    #"Merged Queries1" = Table.NestedJoin(#"Filtered Rows", {"Index"}, #"Aggregated Count", {"Index"}, "Query1", JoinKind.LeftOuter),
    #"Expanded Query1" = Table.ExpandTableColumn(#"Merged Queries1", "Query1", {"Max of Count.rate.rateperiod"}, {"Query1.Max of Count.rate.rateperiod"}),
    #"Added Conditional Column1" = Table.AddColumn(#"Expanded Query1", "condition 2", each if [rate.rateperiod] = [Query1.Max of Count.rate.rateperiod] then [rate.rate] else null),
    #"Filtered Rows1" = Table.SelectRows(#"Added Conditional Column1", each [condition 2] <> null and [condition 2] <> ""),
    #"Removed Columns" = Table.RemoveColumns(#"Filtered Rows1",{"rate.uniquekey", "rate.rateperiod", "rate.rate", "condition1", "Query1.Max of Count.rate.rateperiod"}),
    #"Added Custom" = Table.AddColumn(#"Removed Columns", "new column", each [volume]*[condition 2])
in
    #"Added Custom"