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[ADD] accounting: Impact on the average price valuation when returning goods

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https://docs.google.com/document/d/1PsCT28I8zD4los7uivSoWFe0gYbtuWb6tuwja9QL1J4/edit
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=========================================================================
Impact on the average price valuation when returning goods to a supplier
=========================================================================
==========================================================
Impact on the average price valuation when returning goods
==========================================================
As stated in the `*inventory valuation
page* <https://www.odoo.com/documentation/functional/valuation.html>`__,
one of the possible costing method you can use in perpetual stock
valuation, is the average cost.
This document answers to one recurrent question for companies using that
method to make their stock valuation: how does a shipping returned to
its supplier impact the average cost and the accounting entries? This
document is **only** for the specific use case of a perpetual valuation (as
opposed to the periodic one) and in average price costing method (as
opposed to standard of FIFO).
Definition of average cost
==========================
The average cost method calculates the cost of ending inventory and cost
of goods sold on the basis of weighted average cost per unit of
inventory.
The weighted average cost per unit is calculated using the following
formula:
- When new products arrive in a warehouse, the new average cost is
recomputed as:
.. image:: media/avg01.png
:align: center
- When products leave the warehouse: the average cost **does not** change
Defining the purchase price
---------------------------
The purchase price is estimated at the reception of the products (you
might not have received the vendor bill yet) and reevaluated at the
reception of the vendor bill. The purchase price includes the cost you
pay for the products, but it may also includes additional costs, like
landed costs.
Average cost example
====================
+-----------------------------+---------------+-------------------+---------------+------------+
| Operation | Delta Value | Inventory Value | Qty On Hand | Avg Cost |
+=============================+===============+===================+===============+============+
| | | $0 | 0 | $0 |
+-----------------------------+---------------+-------------------+---------------+------------+
| Receive 8 Products at $10 | +8\*$10 | $80 | 8 | $10 |
+-----------------------------+---------------+-------------------+---------------+------------+
| Receive 4 Products at $16 | +4\*$16 | $144 | 12 | $12 |
+-----------------------------+---------------+-------------------+---------------+------------+
| Deliver 10 Products | -10\*$12 | $24 | 2 | $12 |
+-----------------------------+---------------+-------------------+---------------+------------+
+-----------------------------+---------------+-------------------+---------------+------------+
At the beginning, the Avg Cost is set to 0 set as there is no product in
the inventory. When the first reception is made, the average cost
becomes logically the purchase price.
At the second reception, the average cost is updated because the total
inventory value is now ``$80 + 4*$16 = $144``. As we have 12 units on
hand, the average price per unit is ``$144 / 12 = $12``.
By definition, the delivery of 10 products does not change the average
cost. Indeed, the inventory value is now $24 as we have only 2 units
remaining of each ``$24 / 2 = $12``.
Purchase return use case
========================
In case of a product returned to its supplier after reception, the
inventory value is reduced using the average cost formulae (not at the
initial price of these products!).
Which means that the above table will be updated as follow:
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Operation | Delta Value | Inventory Value | Qty On Hand | Avg Cost |
+===============================================+===============+===================+===============+============+
| | | $24 | 2 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Return of 1 Product initially bought at $10 | -1\*$12 | $12 | 1 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
Explanation: counter example
----------------------------
Remember the definition of **Average Cost**, saying that we do not update
the average cost of a product leaving the inventory. If you break this
rule, you may lead to inconsistencies in your inventory.
As an example, here is the scenario when you deliver one piece to the
customer and return the other one to your supplier (at the cost you
purchased it). Here is the operation:
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Operation | Delta Value | Inventory Value | Qty On Hand | Avg Cost |
+===============================================+===============+===================+===============+============+
| | | $24 | 2 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Customer Shipping 1 product | -1\*$12 | $12 | 1 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Return of 1 Product initially bought at $10 | -1\*$10 | **$2** | **0** | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
As you can see in this example, this is not correct: an inventory
valuation of $2 for 0 pieces in the warehouse.
The correct scenario should be to return the goods at the current
average cost:
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Operation | Delta Value | Inventory Value | Qty On Hand | Avg Cost |
+===============================================+===============+===================+===============+============+
| | | $24 | 2 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Customer Shipping 1 product | -1\*$12 | $12 | 1 | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
| Return of 1 Product initially bought at $10 | -1\*$12 | **$0** | **0** | $12 |
+-----------------------------------------------+---------------+-------------------+---------------+------------+
On the other hand, using the average cost to value the return ensure a
correct inventory value at all times.
Further thoughts on anglo saxon mode
------------------------------------
For people in using the **anglo saxon accounting** principles, there is
another concept to take into account: the stock input account of the
product, which is intended to hold at any time the value of vendor bills
to receive. So the stock input account will increase on reception of
incoming shipments and will decrease when receiving the related vendor
bills.
Back to our example, we see that when the return is valued at the
average price, the amount booked in the stock input account is the
original purchase price:
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Operation | stock input | price diff | Inventory Value | Qty On Hand | Avg Cost |
+===============================================+===============+==============+===================+===============+============+
| | | | $0 | 0 | $0 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Receive 8 Products at $10 | ($80) | | $80 | 8 | $10 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Receive vendor bill $80 | $0 | | $80 | 8 | $10 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Receive 4 Products at $16 | ($64) | | $144 | 12 | $12 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Receive vendor bill $64 | $0 | | $144 | 12 | $12 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Deliver 10 Products | $0 | | $24 | 2 | $12 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Return of 1 Product initially bought at $10 | **$10** | **$2** | **$12** | 1 | $12 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
| Receive vendor refund $10 | $0 | $2 | $12 | 1 | $12 |
+-----------------------------------------------+---------------+--------------+-------------------+---------------+------------+
This is because the vendor refund will be made using the original
purchase price, so to zero out the effect of the return in the stock
input in last operation, we need to reuse the original price. The price
difference account located on the product category is used to book the
difference between the average cost and the original purchase price.

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