Fixed issue 19 by jimboman77: https://github.com/spesmilo/electrum/issues/19

"Right now the only check thats being done is making sure that the length isn't 0, ie something is being entered before the send button becomes clickable."

Also check that entered amount is <= btc_balance before enabling send button.

lib/gui_lite.py

@@ -332,10 +332,14 @@ class MiniWindow(QDialog):
     def check_button_status(self):
         """Check that the bitcoin address is valid and that something
         is entered in the amount before making the send button clickable."""
+        try:
+            value = D(str(self.amount_input.text())) * 10**8
+        except decimal.InvalidOperation:
+            value = None
         # self.address_input.property(...) returns a qVariant, not a bool.
         # The == is needed to properly invoke a comparison.
         if (self.address_input.property("isValid") == True and
-            len(self.amount_input.text()) > 0):
+            value is not None and 0 < value <= self.btc_balance):
             self.send_button.setDisabled(False)
         else:
             self.send_button.setDisabled(True)