Add Values in a Column by matching conditions in other Columns and Rows

let
    fx = (x) => 
        [sorted_list = List.Buffer(Table.ToList(Table.Sort(x, "idx"), (x) => x)),
        rt = List.Generate(
            () => [i = 0, r = sorted_list{0}, net = List.Sum({r{2}, - r{3}})],
            (x) => x[i] < List.Count(sorted_list),
            (x) => [i = x[i] + 1, r = sorted_list{i}, net = List.Sum({x[net], r{2}, - r{3}})],
            (x) => x[r] & {x[net]}
        )
        ][rt],
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    idx = Table.AddIndexColumn(Source, "idx"),
    blank_to_nulls = Table.TransformColumns(idx, {}, (x) => if x = " " then null else x),
    group = Table.Group(blank_to_nulls, "Item Code", {"x", fx}),
    result = Table.RemoveColumns(
        Table.Sort(
            Table.FromList(List.Combine(group[x]), (x) => x, Table.ColumnNames(Source) & {"idx", "NET"}),
            "idx"
        ),
        "idx"
    )
in
    result