invoices: don't modify .amount_msat directly

electrum/invoices.py

@@ -178,6 +178,19 @@ class BaseInvoice(StoredObject):
             return amount_msat
         return int(amount_msat // 1000)
 
+    def set_amount_msat(self, amount_msat: Union[int, str]) -> None:
+        """The GUI uses this to fill the amount for a zero-amount invoice."""
+        if amount_msat == "!":
+            amount_sat = amount_msat
+        else:
+            assert isinstance(amount_msat, int), f"{amount_msat=!r}"
+            assert amount_msat >= 0, amount_msat
+            amount_sat = (amount_msat // 1000) + int(amount_msat % 1000 > 0)  # round up
+        if outputs := self.outputs:
+            assert len(self.outputs) == 1, len(self.outputs)
+            self.outputs = [PartialTxOutput(scriptpubkey=outputs[0].scriptpubkey, value=amount_sat)]
+        self.amount_msat = amount_msat
+
     @amount_msat.validator
     def _validate_amount(self, attribute, value):
         if value is None: