qt send tab: (fix) allow user to set lower fees if "not enough funds"

Previously if the user tried to pay an invoice, we tried to construct
a tx with the desired feerate. If this raise NotEnoughFunds, we would just
show the error and not let the user change the feerate.

related: https://github.com/spesmilo/electrum/issues/6136#issuecomment-622254754 (method 2)

electrum/gui/qt/confirm_tx_dialog.py

@@ -104,7 +104,13 @@ class TxEditor:
         if use_rbf:
             self.tx.set_rbf(True)
 
-
+    def have_enough_funds_assuming_zero_fees(self) -> bool:
+        try:
+            tx = self.make_tx(0)
+        except NotEnoughFunds:
+            return False
+        else:
+            return True
 
 
 

electrum/gui/qt/main_window.py

@@ -1594,8 +1594,11 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger):
         output_value = '!' if '!' in output_values else sum(output_values)
         d = ConfirmTxDialog(window=self, make_tx=make_tx, output_value=output_value, is_sweep=is_sweep)
         if d.not_enough_funds:
-            self.show_message(_('Not Enough Funds'))
+            # Check if we had enough funds excluding fees,
-            return
+            # if so, still provide opportunity to set lower fees.
+            if not d.have_enough_funds_assuming_zero_fees():
+                self.show_message(_('Not Enough Funds'))
+                return
         cancelled, is_send, password, tx = d.run()
         if cancelled:
             return