diff --git a/app/controllers/transactions_controller.rb b/app/controllers/transactions_controller.rb
index 6db16fd..2aad2b2 100644
--- a/app/controllers/transactions_controller.rb
+++ b/app/controllers/transactions_controller.rb
@@ -23,14 +23,14 @@ class TransactionsController < ApplicationController
 
   def transaction_params
     t = params.require(:transaction)
-          .permit(:debtor, :creditor, :message, :euros, :cents)
+          .permit(:debtor, :creditor, :message, :euros, :cents, :id_at_client)
 
     {
       debtor: t[:debtor] ? User.find_or_create_by(name: t[:debtor]) : User.zeus,
       creditor: t[:creditor] ? User.find_or_create_by(name: t[:creditor]) : User.zeus,
       issuer: current_client || current_user,
       amount: (t[:euros].to_f * 100 + t[:cents].to_f).to_i,
-      message: t[:message]
-    }
+      message: t[:message],
+    }.merge(current_client ? { id_at_client: t[:id_at_client] } : {})
   end
 end
diff --git a/app/models/transaction.rb b/app/models/transaction.rb
index 34746ef..7d82df8 100644
--- a/app/models/transaction.rb
+++ b/app/models/transaction.rb
@@ -2,15 +2,16 @@
 #
 # Table name: transactions
 #
-#  id          :integer          not null, primary key
-#  debtor_id   :integer          not null
-#  creditor_id :integer          not null
-#  issuer_id   :integer          not null
-#  issuer_type :string           not null
-#  amount      :integer          default(0), not null
-#  message     :string
-#  created_at  :datetime         not null
-#  updated_at  :datetime         not null
+#  id           :integer          not null, primary key
+#  debtor_id    :integer          not null
+#  creditor_id  :integer          not null
+#  issuer_id    :integer          not null
+#  issuer_type  :string           not null
+#  amount       :integer          default(0), not null
+#  message      :string
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#  id_at_client :integer
 #
 
 class Transaction < ActiveRecord::Base
@@ -22,6 +23,7 @@ class Transaction < ActiveRecord::Base
   after_destroy :recalculate_balances
 
   validates :amount, numericality: { greater_than: 0 }
+  validates :id_at_client, presence: true, uniqueness: { scope: :issuer_id }, if: :is_client_transaction?
   validate :different_debtor_creditor
 
 
@@ -52,4 +54,8 @@ class Transaction < ActiveRecord::Base
       self.errors.add :base, "Can't write money to yourself"
     end
   end
+
+  def is_client_transaction?
+    issuer_type == "Client"
+  end
 end
diff --git a/db/migrate/20150914095049_add_id_at_client_to_transactions.rb b/db/migrate/20150914095049_add_id_at_client_to_transactions.rb
new file mode 100644
index 0000000..a2ceea8
--- /dev/null
+++ b/db/migrate/20150914095049_add_id_at_client_to_transactions.rb
@@ -0,0 +1,6 @@
+class AddIdAtClientToTransactions < ActiveRecord::Migration
+  def change
+    add_column :transactions, :id_at_client, :int
+    add_index :transactions, [:issuer_id, :id_at_client]
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index f6bf104..d784c74 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20150908092731) do
+ActiveRecord::Schema.define(version: 20150914095049) do
 
   create_table "clients", force: :cascade do |t|
     t.string   "name",       null: false
@@ -24,18 +24,20 @@ ActiveRecord::Schema.define(version: 20150908092731) do
   add_index "clients", ["name"], name: "index_clients_on_name"
 
   create_table "transactions", force: :cascade do |t|
-    t.integer  "debtor_id",               null: false
-    t.integer  "creditor_id",             null: false
-    t.integer  "issuer_id",               null: false
-    t.string   "issuer_type",             null: false
-    t.integer  "amount",      default: 0, null: false
+    t.integer  "debtor_id",                null: false
+    t.integer  "creditor_id",              null: false
+    t.integer  "issuer_id",                null: false
+    t.string   "issuer_type",              null: false
+    t.integer  "amount",       default: 0, null: false
     t.string   "message"
-    t.datetime "created_at",              null: false
-    t.datetime "updated_at",              null: false
+    t.datetime "created_at",               null: false
+    t.datetime "updated_at",               null: false
+    t.integer  "id_at_client"
   end
 
   add_index "transactions", ["creditor_id"], name: "index_transactions_on_creditor_id"
   add_index "transactions", ["debtor_id"], name: "index_transactions_on_debtor_id"
+  add_index "transactions", ["issuer_id", "id_at_client"], name: "index_transactions_on_issuer_id_and_id_at_client"
   add_index "transactions", ["issuer_type", "issuer_id"], name: "index_transactions_on_issuer_type_and_issuer_id"
 
   create_table "users", force: :cascade do |t|
diff --git a/spec/apis/transactions_controller_spec.rb b/spec/apis/transactions_controller_spec.rb
index d15aec1..9acc373 100644
--- a/spec/apis/transactions_controller_spec.rb
+++ b/spec/apis/transactions_controller_spec.rb
@@ -3,11 +3,12 @@ describe TransactionsController, type: :api do
     @debtor   = create :user
     @creditor = create :user
     @api_attributes = {
-      debtor:   @debtor.name,
-      creditor: @creditor.name,
-      message:  Faker::Lorem.sentence,
-      euros:    1,
-      cents:    25
+      debtor:       @debtor.name,
+      creditor:     @creditor.name,
+      message:      Faker::Lorem.sentence,
+      euros:        1,
+      cents:        25,
+      id_at_client: 1
     }
     @client = Client.create name: "Tap"
     @key = @client.key
diff --git a/spec/factories/transactions.rb b/spec/factories/transactions.rb
index 71e7e00..237b789 100644
--- a/spec/factories/transactions.rb
+++ b/spec/factories/transactions.rb
@@ -2,15 +2,16 @@
 #
 # Table name: transactions
 #
-#  id          :integer          not null, primary key
-#  debtor_id   :integer          not null
-#  creditor_id :integer          not null
-#  issuer_id   :integer          not null
-#  issuer_type :string           not null
-#  amount      :integer          default(0), not null
-#  message     :string
-#  created_at  :datetime         not null
-#  updated_at  :datetime         not null
+#  id           :integer          not null, primary key
+#  debtor_id    :integer          not null
+#  creditor_id  :integer          not null
+#  issuer_id    :integer          not null
+#  issuer_type  :string           not null
+#  amount       :integer          default(0), not null
+#  message      :string
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#  id_at_client :integer
 #
 
 FactoryGirl.define do
@@ -20,5 +21,9 @@ FactoryGirl.define do
     issuer { debtor }
     amount { 1 + rand(100) }
     message { Faker::Lorem.sentence }
+    factory :client_transaction do
+      association :issuer, factory: :client
+      sequence :id_at_client
+    end
   end
 end
diff --git a/spec/helpers/pages_helper_spec.rb b/spec/helpers/pages_helper_spec.rb
deleted file mode 100644
index 2960941..0000000
--- a/spec/helpers/pages_helper_spec.rb
+++ /dev/null
@@ -1,15 +0,0 @@
-require 'rails_helper'
-
-# Specs in this file have access to a helper object that includes
-# the PagesHelper. For example:
-#
-# describe PagesHelper do
-#   describe "string concat" do
-#     it "concats two strings with spaces" do
-#       expect(helper.concat_strings("this","that")).to eq("this that")
-#     end
-#   end
-# end
-RSpec.describe PagesHelper, type: :helper do
-  pending "add some examples to (or delete) #{__FILE__}"
-end
diff --git a/spec/models/transaction_spec.rb b/spec/models/transaction_spec.rb
index d3cc92c..bed16fc 100644
--- a/spec/models/transaction_spec.rb
+++ b/spec/models/transaction_spec.rb
@@ -2,15 +2,16 @@
 #
 # Table name: transactions
 #
-#  id          :integer          not null, primary key
-#  debtor_id   :integer          not null
-#  creditor_id :integer          not null
-#  issuer_id   :integer          not null
-#  issuer_type :string           not null
-#  amount      :integer          default(0), not null
-#  message     :string
-#  created_at  :datetime         not null
-#  updated_at  :datetime         not null
+#  id           :integer          not null, primary key
+#  debtor_id    :integer          not null
+#  creditor_id  :integer          not null
+#  issuer_id    :integer          not null
+#  issuer_type  :string           not null
+#  amount       :integer          default(0), not null
+#  message      :string
+#  created_at   :datetime         not null
+#  updated_at   :datetime         not null
+#  id_at_client :integer
 #
 
 describe Transaction, type: :model do
@@ -50,4 +51,25 @@ describe Transaction, type: :model do
       expect(build :transaction, debtor: @user, creditor: @user).to_not be_valid
     end
   end
+
+  describe "client as issuer" do
+    before :each do
+      @transaction = create :client_transaction
+    end
+
+    it "should have a id_at_client" do
+      @transaction.id_at_client = nil
+      expect(@transaction).to_not be_valid
+    end
+
+    it "should have a unique id_at_client" do
+      t = build :client_transaction, issuer: @transaction.issuer, id_at_client: @transaction.id_at_client
+      expect(t).to_not be_valid
+    end
+
+    it "should have a unique id_at_client per client" do
+      t = build :client_transaction, id_at_client: @transaction.id_at_client
+      expect(t).to be_valid
+    end
+  end
 end