[FW][FIX] stock_account,purchase_stock: compensate return of fifo product

[fw-bot]

The value of the returned product may be different from the one
initially received. In such case, the stock accounting may be broken.

To reproduce the issue:
(Need account_accountant)

1. Create an auto-FIFO product category
2. Create a storable product
3. Confirm a PO with 1 @ 10
4. Receive it
5. Confirm a PO with 4 @ 25
6. Receive 1 with backorder
7. Return it
8. On the backorder, receive 4
9. Bill

Error: Looking at the AML of the stock-in account, some lines are
not reconciled and there is a $15 difference between the totald debit
and the total credit.

Step 7, when returning the product, we actually return the one at
$10. Step 8, we then receive 4 products at $25 (hence the difference
of $15). So, back to step 7, when returning the product, we should
also compensate the stock-in account (with the expense one) in case
of a difference.

Let's look at another case: after step 7, the user "returns the
return". In such case, the value of the new receipt is based on the
returned one (see [1]), i.e.: the value of the newly received
product will be $10. In such case, we also need to (1) compensate
the stock-in account and (2) prevent the pdiff process to generate a
pdiff of $15 (otherwise, we would add a value to the received
product, which would go against the logic of [1]).

This commit does not address the behaviour difference between the
two above use cases (-> depending on how the user receives again the
returned product, its value is not the same). This would probably
need a deeper analysis a change, on master.

Note: a third use case does not work either: (step 1-4), PO 1 @ 25,
receive, bill, return, refund.

[1]
https://github.com/odoo/odoo/blob/07464844c69ac3b667adfede0e4e330831819802/addons/purchase_stock/models/stock_move.py#L32-L40
Initially added by 200aac5

OWP-3698324

Forward-Port-Of: #162368

[robodoo]

.

[fw-bot]

This PR targets saas-16.4 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[fw-bot]

@adwid @Whenrow the next pull request (#162697) is in conflict. You can merge the chain up to here by saying

@fw-bot r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[adwid]

robodoo r+