[FIX] purchase: rounding in anglo-saxon

When anglo-saxon accounting is enabled and the valuation of a product is
'real_time', rounding errors might occur in the price difference between
the invoice price and the product price. It occurs when the product
price precision is larger than the accounting precision.

To avoid these rounding errors, we calculate the difference as the
difference between the price before update and the price after update.

opw-686730

addons/purchase/invoice.py

@@ -163,13 +163,14 @@ class AccountInvoice(models.Model):
                                 #line['tax_ids'] is like [(4, tax_id, None), (4, tax_id2, None)...]
                                 taxes = self.env['account.tax'].browse([x[1] for x in line['tax_ids']])
                                 price_unit = taxes.with_context(round=False).compute_all(price_unit, currency=inv.currency_id, quantity=1.0)['total_excluded']
+                            price_before = line.get('price', 0.0)
                             line.update({'price': round(valuation_price_unit * line['quantity'], account_prec)})
                             diff_res.append({
                                 'type': 'src',
                                 'name': i_line.name[:64],
                                 'price_unit': round(price_unit - valuation_price_unit, product_prec),
                                 'quantity': line['quantity'],
-                                'price': round((price_unit - valuation_price_unit) * line['quantity'], account_prec),
+                                'price': round(price_before - line.get('price', 0.0), account_prec),
                                 'account_id': acc,
                                 'product_id': line['product_id'],
                                 'uom_id': line['uom_id'],