diff --git a/python/rest-api/rest_api.py b/python/rest-api/rest_api.py
index 08d4176..d8f2744 100644
--- a/python/rest-api/rest_api.py
+++ b/python/rest-api/rest_api.py
@@ -1,9 +1,12 @@
 import json
 from collections import defaultdict
 from functools import wraps
+from operator import attrgetter
 
 VALID_HTTP_METHODS = ['GET', 'POST']
 
+def dump(obj):
+    return obj.__dict__
 
 class User:
     def __init__(self, name):
@@ -12,18 +15,32 @@ class User:
         self.owed_by = defaultdict(float)
         self.balance = 0.0
 
-    def lend(self, lender_name, amount):
-        self.owed_by[lender_name] += amount
-        if not self.owed_by[lender_name]:
-            del self.owed_by[lender_name]
+    def lend(self, borrower, amount):
 
-    def borrow(self, borrower_name, amount):
-        self.owes[borrower_name] += amount
-        if not self.owed_by[borrower_name]:
-            del self.owed_by[borrower_name]
+        total_owed = self.owed_by[borrower] - self.owes[borrower] + amount
+        if total_owed > 0:
+            self.owed_by[borrower] = total_owed
+            self.owes.pop(borrower, None)
+        elif total_owed < 0:
+            self.owes[borrower] = -total_owed
+            self.owed_by.pop(borrower, None)
+        else:
+            self.owed_by.pop(borrower, None)
+            self.owes.pop(borrower, None)
+
+        self.balance += amount
+
+    def borrow(self, lender, amount):
+        self.lend(lender, -amount)
+
+    @classmethod
+    def from_dict(cls, d):
+        user = cls(d['name'])
+        user.owed_by.update(d['owed_by'])
+        user.owes.update(d['owes'])
+        user.balance = d['balance']
+        return user
 
-    def to_json(self):
-        return json.dumps(self.__dict__)
 
 class Registry:
     def __init__(self):
@@ -45,39 +62,45 @@ class RestAPI:
     registry = Registry()
 
     def __init__(self, database=None):
-        self._database = database if database else {}
-        self._users = {}
+        self._users = {user['name'] : User.from_dict(user) for user in database['users']} if database else {}
 
     @registry.register_url('/users', 'GET')
     def users(self, payload=None):
-        pass
+        if payload:
+            requested_users = json.loads(payload)['users']
+            response = [user for user in self._users.values() if user.name in requested_users]
+        else:
+            response = [user for user in self._users.values()]
+
+        return json.dumps({'users': response}, default=dump, sort_keys=True)
 
     @registry.register_url('/add', 'POST')
     def add(self, payload=None):
-        try:
-            payload = json.loads(payload)
-            user = User(payload['user'])
-            self._users[user.name] = user
-            return user.to_json()
-        except:
-            pass
+        payload = json.loads(payload)
+        user = User(payload['user'])
+        self._users[user.name] = user
+        return json.dumps(user, default=dump)
 
     @registry.register_url('/iou', 'POST')
     def iou(self, payload=None):
-        pass
+        payload = json.loads(payload)
+        lender = payload['lender']
+        borrower = payload['borrower']
+        amount = payload['amount']
+        self._users[lender].lend(borrower, amount)
+        self._users[borrower].borrow(lender, amount)
+        return json.dumps({'users': sorted([self._users[lender], self._users[borrower]], key=attrgetter('name'))}, default=dump, sort_keys=True)
 
     def get(self, url, payload=None):
         methods = self.registry.methods['GET']
         if url in methods.keys():
             return methods[url](self, payload)
-        return ''
-#        else:
-#            raise Exception('Error 404')
+        else:
+            raise Exception('Error 404')
 
     def post(self, url, payload=None):
         methods = self.registry.methods['POST']
         if url in methods.keys():
             return methods[url](self, payload)
-        return ''
-#        else:
-#            raise Exception('Error 404')
+        else:
+            raise Exception('Error 404')