[FIX] point_of_sale: correctly compute cash profit for closing popup

Current behavior:
If you add cash at the opening of a session that already has cash in the
cash register; the closing popup will show a negative profit (loss) for
the cash payment method.

Steps to reproduce:
- Open a POS session
- Add cash to the cash register (50€)
- Close the session
- Open a new session
- The popup show that there is already 50€ in the cash register, add
  cash to the cash register (100€)
- Close the session, the closing popup will show a loss of 50€ for the
  cash payment method. Wich is not true, it's a profit of 50€.

opw-3101297

closes odoo/odoo#108870

X-original-commit: d881de9079bc9d8cab6811872eaa2c2138f6612d
Signed-off-by: Heinz Robin (rhe) <rhe@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>

addons/point_of_sale/models/pos_session.py

@@ -340,7 +340,7 @@ class PosSession(models.Model):
                     'pos_session_id': self.id,
                 }
 
-            if self.cash_register_difference < 0.0:
+            if amount < 0.0:
                 if not self.cash_journal_id.loss_account_id:
                     raise UserError(
                         _('Please go on the %s journal and define a Loss Account. This account will be used to record cash difference.',