[...]
    ReplacedValue = Table.ReplaceValue( 
                        Custom1,
                        0,
                        each [CD],
                        (a,b,c)=> List.Dates( a,
                                            Number.From( (if c=null then Date.From(DateTime.LocalNow()) else c) - a ),
                                            #duration(1,0,0,0) ),
                        {"ContractDate"}),
[...]

let
    Sales     = Excel.CurrentWorkbook(){[Name="Sales"]}[Content][[ID Cust], [Date], [ID Merch], [Rev]],
    Tax       = Table.Buffer(Table.TransformColumnTypes(Excel.CurrentWorkbook(){[Name="Tax"]}[Content],{"ID БН",type number},"")),
    Prefix    = Table.TransformColumns(Tax,{"ID БН", each "БН "&Text.From(_,"")}),
    IDБН      = List.Buffer(List.Transform({1..List.Max(Tax[#"ID БН"])},each "БН "&Text.From(_,""))),
    Indexed   = Table.AddIndexColumn(Sales, "i", 0),
    Joined    = Table.NestedJoin(Indexed,"ID Cust",Prefix,"ID Cust","x"),
    Transform = Table.ReplaceValue(Joined,
                    each [Date],
                    0,
                    (a,b,c)=> let 
                        filtered = Table.SelectRows(a,each [ContractDate]<=b), 
                        grouped  = Table.Group(filtered, {"ID БН"}, 
                                       {"_", each Table.Max(_,"ContractDate"), type []}
                                   ),
                        totable  = Table.FromRecords(grouped[_]),
                        Typed    = Table.TransformColumnTypes(totable, {"Rate", Percentage.Type},""),
                        pivot    = Table.Pivot(totable, IDБН, "ID БН", "Rate")
                    in pivot,
                    {"x"}
                ),
    Expand    = Table.ExpandTableColumn(Transform, "x", IDБН),
    Typed     = Table.TransformColumnTypes(Expand, 
                    {{"Date",type date}}&
                    List.Zip({
                        IDБН&{"ID Cust","ID Merch","Rev"},
                        List.Repeat({type number},List.Count(IDБН)+3)
                    }),
                    ""
                ),
    Sorted    = Table.Sort(Typed,{{"i", Order.Ascending}}),
    RemoveCol = Table.RemoveColumns(Sorted,{"i"})
in
    RemoveCol