diff --git a/htdocs/compta/facture/class/api_invoices.class.php b/htdocs/compta/facture/class/api_invoices.class.php
index 9738cec55c1..54f47dc87db 100644
--- a/htdocs/compta/facture/class/api_invoices.class.php
+++ b/htdocs/compta/facture/class/api_invoices.class.php
@@ -667,6 +667,49 @@ class Invoices extends DolibarrApi
     }
 
 
+     /**
+     * Insert a discount in a specific invoice
+     *
+     * @param int   $id             Id of invoice
+     * @param int   $discountId     Id of discount
+     *
+     * @url     POST {id}/adddiscount/{discountId}
+     *
+     * @return int
+     * @throws 400
+     * @throws 401
+     * @throws 404
+     * @throws 405
+     */
+    function addDiscount($id, $discountId) {
+
+        if(! DolibarrApiAccess::$user->rights->facture->creer) {
+                throw new RestException(401);
+        }
+        if(empty($id)) {
+                throw new RestException(400, 'Invoice ID is mandatory');
+        }
+        if(empty($discountId)) {
+                throw new RestException(400, 'Discount ID is mandatory');
+        }
+
+        if( ! DolibarrApi::_checkAccessToResource('facture',$id)) {
+                throw new RestException(401, 'Access not allowed for login '.DolibarrApiAccess::$user->login);
+        }
+
+        $result = $this->invoice->fetch($id);
+        if( ! $result ) {
+                throw new RestException(404, 'Invoice not found');
+        }
+
+        $result = $this->invoice->insert_discount($discountId);
+        if( $result < 0) {
+                throw new RestException(405, $this->invoice->error);
+        }
+
+        return $result;
+    }
+
     /**
      * Get a payment list of a given invoice
      *