This article will explain how to reject expenses.

As an Approver, Controller or Finance, you can reject expense(s). Once you click the Reject tab in the Expense overview, or within the expense, a pop-up message will appear. 

Within the pop-up you can select the reject reason in the drop-down list and you can add a comment (e.g. what should be changed for the expense to be approved) in the message box.