don't withdraw small amounts with bigger gas price

fixes #4773

golem/ethereum/transactionsystem.py

@@ -634,6 +634,9 @@ def withdraw(
         if not is_address(destination):
             raise ValueError("{} is not valid ETH address".format(destination))
 
+        if gas_price and amount < gas_price:
+            raise Exception("Gas price is higer than amount")
+
         log.info(
             "Trying to withdraw %f %s to %s",
             amount / denoms.ether,

tests/golem/ethereum/test_transactionsystem.py

@@ -393,7 +393,15 @@ def setUp(self):
 
         self.eth_tx = f'0x{"e"*64}'
         self.gntb_tx = f'0x{"f"*64}'
-        self.sci.transfer_eth.return_value = self.eth_tx
+
+        def transfer_eth(
+                to_address: str,
+                amount: int,
+                gas_price: Optional[int] = None) -> str:
+            assert amount > 0
+            return self.eth_tx
+
+        self.sci.transfer_eth.side_effect = transfer_eth
         self.sci.convert_gntb_to_gnt.return_value = self.gntb_tx
 
         self.ets._refresh_balances()
@@ -454,6 +462,13 @@ def test_custom_gas_price_eth(self):
             gas_price,
         )
 
+    def test_gas_price_higher_than_amount(self):
+        amount = denoms.ether
+        gas_price = amount+1
+        with self.assertRaisesRegex(Exception,
+                                    "Gas price is higer than amount"):
+            self.ets.withdraw(amount, self.dest, 'ETH', gas_price)
+
     def test_eth_with_lock(self):
         self.ets.lock_funds_for_payments(1, 1)
         locked_eth = self.ets.get_locked_eth()