FIFO Cost

Power Query may not be the best solution, but I made it work

let
    Purchase = Excel.CurrentWorkbook(){[Name="Purchase"]}[Content],
    Sales = Excel.CurrentWorkbook(){[Name="Sales"]}[Content],
    Custom1 = 
        Table.AddColumn(
            Table.Group( Sales, "Product Name", { "S", each _ } ),
            "P",
            (x)=> List.Select( Table.ToRows( Purchase ), (y)=> y{1} = x[Product Name] )
        ),
    Custom2 = 
        Table.ToList(
            Custom1,
            (t)=>
                let
                    f =
                        (p)=>
                        List.Accumulate(
                            t{2},
                            { 0, 0 },
                            (x,y)=>
                                if x{1} + y{2} < p
                                then { x{0} + y{2} * y{3}, x{1} + y{2} }
                                else { x{0} + List.Max( { 0, p - x{1} } ) * y{3}, x{1} + y{2} }
                        ){0},
                    a =
                        Table.AddIndexColumn( t{1}, "i", 1, 1 ),
                    b =
                        Table.AddColumn( a, "q", each f( List.Sum( Table.FirstN( a, [i] )[Quantity] ) ) ),
                    c =
                        Table.AddColumn( b, "amount", each if [i] = 1 then [q] else [q] - b[q]{[i]-2} ),
                    l =
                        List.Transform( List.Zip( List.Transform( t{2}, (x)=> { x{2}, x{2} * x{3} } ) ), List.Sum ),
                    d =
                        Table.AddColumn( c, "Qty", each l{0} - List.Sum( Table.FirstN( c, [i] )[Quantity] ) ),
                    e =
                        Table.AddColumn( d, "Value", each l{1} - List.Sum( Table.FirstN( c, [i] )[amount] ) )
                in
                    Table.RemoveColumns( e, { "i", "q" } )
        ),
    Custom3 = 
        Table.Combine( Custom2 )
in
    Custom3