[FIX] stock_account: no amount currency on correction layer for discount

Usecase to reproduce:
- Set stock valuation as perpetual
- Create a PO with a different currency than the company
- Validate the receipt on create the invoice
- Set a discount on the invoice and validate it

The amount currency is not set on the account.move.line created by the
valuation layer. It could prevent the reconciliation to succeed since
it would be base on invoice currency.

It happens because the amount currency is created from the difference
between layer price converted to invoice currency and the invoice line
`price_unit`. However `price_unit` is missing the discount. So this pr
add extra computation to get the gross price unit discount applied.

closes odoo/odoo#107212

Signed-off-by: William Henrotin (whe) <whe@odoo.com>

addons/stock_account/models/account_move.py

@@ -290,16 +290,7 @@ class AccountMoveLine(models.Model):
             if not layers:
                 continue
 
-            price_unit = -line.price_unit if move.move_type == 'in_refund' else line.price_unit
-            price_unit = price_unit * (1 - (line.discount or 0.0) / 100.0)
-            if line.tax_ids:
-                prec = 1e+6
-                price_unit *= prec
-                price_unit = line.tax_ids.with_context(round=False).compute_all(
-                    price_unit, currency=move.currency_id, quantity=1.0, is_refund=move.move_type == 'in_refund',
-                    fixed_multiplicator=move.direction_sign,
-                )['total_excluded']
-                price_unit /= prec
+            price_unit = line._get_gross_unit_price()
             price_unit = line.currency_id._convert(price_unit, line.company_id.currency_id, line.company_id, line.date, round=False)
             price_unit = line.product_uom_id._compute_price(price_unit, line.product_id.uom_id)
             layers_price_unit = line._get_stock_valuation_layers_price_unit(layers)
@@ -311,6 +302,20 @@ class AccountMoveLine(models.Model):
         self.ensure_one()
         return self.product_id.type == 'product' and self.product_id.valuation == 'real_time'
 
+    def _get_gross_unit_price(self):
+        price_unit = -self.price_unit if self.move_id.move_type == 'in_refund' else self.price_unit
+        price_unit = price_unit * (1 - (self.discount or 0.0) / 100.0)
+        if not self.tax_ids:
+            return price_unit
+        prec = 1e+6
+        price_unit *= prec
+        price_unit = self.tax_ids.with_context(round=False).compute_all(
+            price_unit, currency=self.move_id.currency_id, quantity=1.0, is_refund=self.move_id.move_type == 'in_refund',
+            fixed_multiplicator=self.move_id.direction_sign,
+        )['total_excluded']
+        price_unit /= prec
+        return price_unit
+
     def _get_stock_valuation_layers(self, move):
         valued_moves = self._get_valued_in_moves()
         if move.move_type == 'in_refund':
@@ -353,7 +358,7 @@ class AccountMoveLine(models.Model):
             if float_is_zero(unit_valuation_difference * qty_to_correct, precision_rounding=self.company_id.currency_id.rounding):
                 continue
             po_pu_curr = po_line.currency_id._convert(po_line.price_unit, self.currency_id, self.company_id, self.date, round=False)
-            price_difference_curr = po_pu_curr - self.price_unit
+            price_difference_curr = po_pu_curr - self._get_gross_unit_price()
             layers_to_correct[layer] = (qty_to_correct, unit_valuation_difference, price_difference_curr)
         return layers_to_correct