Use invoice object when crediting an invoice

invoice.py

@@ -160,12 +160,12 @@ class Invoice:
         return result
 
     def _credit(self):
-        res = super(Invoice, self)._credit()
+        credit = super(Invoice, self)._credit()
         for field in _SII_INVOICE_KEYS:
-            res[field] = getattr(self, field)
+            setattr(credit, field, getattr(self, field))
 
-        res['sii_operation_key'] = 'R4'
-        return res
+        credit.sii_operation_key = 'R4'
+        return credit
 
     def _set_sii_keys(self):
         tax = self.taxes and self.taxes[0]

tests/scenario_aeat_sii.rst

@@ -194,3 +194,16 @@ Create AEAT Report::
     >>> report.click('load_invoices')
     >>> len(report.lines)
     2
+
+
+Credit invoice with refund::
+
+    >>> credit = Wizard('account.invoice.credit', [invoice])
+    >>> credit.form.with_refund = True
+    >>> credit.execute('credit')
+    >>> invoice.reload()
+    >>> invoice.state
+    'paid'
+    >>> credit, = Invoice.find([('total_amount', '<', 0)])
+    >>> credit.sii_operation_key
+    'R1'