[trunk] [stock] return to suppliers does not update average cost see also610738

If you return products, the return picks up the cost of the original receipt. That is all fine and correct.

However it does not update average cost when processing leading to stock valuation errors. see the this bug for more discussion - 610738

You receive 5 items at $6 and 5 items at $4

So in stock 10 items at average $5 your stock valuation is $50.

You return 5 items at $6.
Correct Result
Stock Valuation Account - $30
Stock Output Account + $30
5 items at average $4 $20.
OpenERP Result
Stock Valuation Account - $30
Stock Output Account + $30
5 items at average $5 $25.

So our total stock value has decreased $30 but our products stock value only $25. Note the only error is the failure to recompute the products average cost. This error is then populated downstream when products are sold.

This is wrong. Average cost must be updated on every transaction where the cost price of the goods coming in or out is different to the average cost. In practice, shipments to customers should always be at average cost, so we need not worry here, although it would be good practice to recompute here also (or raise an error), but shipments to suppliers could be at any cost and not necessarily the products average cost, therefore it needs recalculating.