diff --git a/Sales/Invoice.cs b/Sales/Invoice.cs
--- a/Sales/Invoice.cs
+++ b/Sales/Invoice.cs
@@ -1,9 +1,28 @@
 /**
  * Invoice.cs
  * 
- * @author       Markus Bröker<broeker.markus@googlemail.com>
- * @copyright    Copyright(C) 2017 4Customers UG
- * 
+ * The MIT License (MIT)
+ *
+ * @copyright Copyright(C) 2017 4Customers UG
+ * @author    Markus Bröker<broeker.markus@googlemail.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
  */
 using System;
 using System.Collections.Generic;