Previous and Current month sales together

Source data left and result right

Solution:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}, {"ItemID", type text}, {"Sales", Int64.Type}}),
    // Assuming the most recent date to be in the current month. Change accroing to your use case.
    current_month = Date.StartOfMonth(List.Max(Source[Date])),
    previous_month = Date.AddMonths(current_month, -1),
    #"Added Month" = Table.AddColumn(Source, "Month", each let 
                month = Date.StartOfMonth([Date])
            in
                if month = current_month then "CurrentMonthSale" 
                else if month = previous_month then "PreviousMonthSale"
                else null),
    #"Removed Date" = Table.RemoveColumns(#"Added Month",{"Date"}),
    // Just in case there are more than just Current Month and Previous Month sales in the data (this is not the case in the test data set)
    #"Filtered out other months" = Table.SelectRows(#"Removed Date", each [Month] <> null),
    #"Pivoted Sales by Month (SUM)" = Table.Pivot(#"Filtered out other months", List.Distinct(#"Filtered out other months"[Month]), "Month", "Sales", List.Sum),
    // Note the ?? operator: A ?? B will return if A = null then B else A
    #"Grouped Rows" = Table.Group(#"Pivoted Sales by Month (SUM)", {"ItemID"}, {{"PreviousMonthSale", each List.Sum([PreviousMonthSale]) ?? 0, type nullable number}, {"CurrentMonthSale", each List.Sum([CurrentMonthSale]) ?? 0, type nullable number}}),
    #"Added Difference" = Table.AddColumn(#"Grouped Rows", "Difference", each [CurrentMonthSale]-[PreviousMonthSale])
in
    #"Added Difference"

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Please share a sample of your data...

Single Sales table as you stated in your original post or 2 sales tables as in your sample?

in the example, why for example for the row 2, the prevous mount cale is not equal to the value of current mount sale for row 1?

is there any other thing should be considered?