diff --git a/src/naiback/broker/broker.py b/src/naiback/broker/broker.py
index 2a03759..9d70015 100644
--- a/src/naiback/broker/broker.py
+++ b/src/naiback/broker/broker.py
@@ -21,14 +21,23 @@ class Broker:
     def add_position(self, ticker, price, amount):
         volume = abs(price * amount)
         if amount > 0:
-            if volume * (1 + self.commission_percentage) > self.cash_:
+            if volume * (1 + 0.01 * self.commission_percentage) > self.cash_:
                 return None
         pos = Position(ticker)
         pos.enter(price, amount)
-        self.cash_ -= (volume + volume * self.commission_percentage)
+        self.cash_ -= price * amount
+        self.cash_ -= volume * 0.01 * self.commission_percentage
         self.positions.append(pos)
         return pos
 
+    def close_position(self, pos, price):
+        volume = abs(price * pos.size())
+        size = pos.size()
+        pos.exit(price)
+
+        self.cash_ += price * size
+        self.cash_ -= volume * 0.01 * self.commission_percentage
+
     def set_commission(self, percentage):
         self.commission_percentage = percentage
 
diff --git a/tests/test_broker.py b/tests/test_broker.py
index df08f9b..ce14234 100644
--- a/tests/test_broker.py
+++ b/tests/test_broker.py
@@ -41,3 +41,21 @@ def test_broker_all_position():
 
     assert pos in broker.all_positions()
 
+def test_broker_close_position():
+    broker = Broker(initial_cash=100)
+
+    pos = broker.add_position('FOO', price=10, amount=1)
+    broker.close_position(pos, price=12)
+
+    assert broker.cash() == 100 + 2
+
+def test_broker_close_position_with_commission():
+    broker = Broker(initial_cash=100)
+    
+    broker.set_commission(percentage=1) # 1%
+
+    pos = broker.add_position('FOO', price=10, amount=1)
+    broker.close_position(pos, price=12)
+
+    assert broker.cash() == 100 + 2 - (10 + 12) * 0.01
+