Join two tables

This would be easier in DAX, but it is possible in M too.

A key piece is to create a new column that records the cumulative sum of sales of each item. To do this, refer to this prior thread:
https://community.powerbi.com/t5/Desktop/How-to-do-a-running-Sum-by-group-in-Power-Query/m-p/290123

Supposing you have that, then you can merge with the Stock table to get the QTY and then write some logic to put it all together. Here's an expanded example similar to yours:

Here's the M query you can use to reproduce the above (assuming you have the Stock table you defined as a separate query):

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUXIEEWZAbKAUqxOtZARkBYOEjOBCxshCRmAhE2QhE7CQKcgsENcCrtEMJgQRBgmZw4RAhhpClIE0BBuhWGkJEzKGWwlUjKTMVCk2FgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Sales order no" = _t, #"Item no" = _t, #"QTY on sales order" = _t, PrevSales = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Sales order no", Int64.Type}, {"QTY on sales order", Int64.Type}, {"PrevSales", Int64.Type}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"Item no"}, Stock, {"Item No"}, "Stock", JoinKind.LeftOuter),
    #"Expanded Stock" = Table.ExpandTableColumn(#"Merged Queries", "Stock", {"QTY"}, {"QTY"}),
    #"Added Custom1" = Table.AddColumn(#"Expanded Stock", "QTY On Stock", each
        if [QTY on sales order] + [PrevSales] > [QTY]
        then (if [QTY] > [PrevSales] then [QTY] - [PrevSales] else 0)
        else [QTY on sales order], Int64.Type)
in
    #"Added Custom1"

Note that this does not include the running total logic.