[FIX] purchase: change invoice date

- Create a PO, generate the vendor bill
- Change a price unit
- Change the invoice date

The price unit is reset to the PO value.

We only recompute it if the currency of the invoice is different from
the currency of the PO.

Fixes odoo#22164
opw-804894

addons/purchase/models/account_invoice.py

@@ -89,7 +89,9 @@ def purchase_order_change(self):
     def _onchange_currency_id(self):
         if self.currency_id:
             for line in self.invoice_line_ids.filtered(lambda r: r.purchase_line_id):
-                line.price_unit = line.purchase_id.currency_id.with_context(date=self.date_invoice).compute(line.purchase_line_id.price_unit, self.currency_id, round=False)
+                if line.purchase_id.currency_id != self.currency_id:
+                    line.price_unit = line.purchase_id.currency_id.with_context(date=self.date_invoice)\
+                        .compute(line.purchase_line_id.price_unit, self.currency_id, round=False)
 
     @api.onchange('invoice_line_ids')
     def _onchange_origin(self):