Subtract previous days using M-Query

(In haste) not especially efficient:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjDUByIjAyMTJR0lRyA2NjBQitVBk3ACYhOYhBGaDkMDbDIgLaYwCWN0LWbYZEBaLEASsQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, Category = _t, Sales = _t]),
    fnMonthlyActualSales = (tbl)=>
    let
        SortedRows = Table.Sort(tbl,{{"Date", Order.Ascending}}),
        AddedIndex = Table.AddIndexColumn(SortedRows, "Index.1", -1, 1, Int64.Type),
        AddedCustom = Table.AddColumn(AddedIndex, "Actual Sales", each [Sales] - (if [Index.1]<0 then 0 else AddedIndex[Sales]{[Index.1]})),
        RemovedOtherColumns = Table.SelectColumns(AddedCustom,{"Date", "Category", "Sales", "Actual Sales", "Index"})
    in
        RemovedOtherColumns,
    ChangedType = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Category", type text}, {"Sales", type number}}),
    AddedIndex = Table.AddIndexColumn(ChangedType, "Index", 0, 1, Int64.Type),
    InsertedEOM = Table.AddColumn(AddedIndex, "End of Month", each Date.EndOfMonth([Date]), type date),
    GroupedRows = Table.Group(InsertedEOM, {"Category", "End of Month"}, {{"grp", each _, type table [Date=nullable date, Category=text, Sales=number, End of Month=date]}}),
    InvokedCustomFunction = Table.AddColumn(GroupedRows, "MonthlyActualSales", each fnMonthlyActualSales([grp])),
    RemovedColumns = Table.RemoveColumns(InvokedCustomFunction,{"grp", "End of Month"}),
    ExpandedMonthlyActualSales = Table.ExpandTableColumn(RemovedColumns, "MonthlyActualSales", {"Date", "Sales", "Actual Sales", "Index"}, {"Date", "Sales", "Actual Sales", "Index"}),
    SortedRows = Table.Sort(ExpandedMonthlyActualSales,{{"Index", Order.Ascending}}),
    ReorderedColumns = Table.ReorderColumns(SortedRows,{"Date", "Category", "Sales", "Actual Sales", "Index"}),
    RemovedColumns1 = Table.RemoveColumns(ReorderedColumns,{"Index"}),
    ChangedType1 = Table.TransformColumnTypes(RemovedColumns1,{{"Date", type date}})
in
    ChangedType1

It treats the data on a monthly basis, so assumes first sale per month is not a cumulative figure.