To reject an expense report, click on the down arrow next to the Approve button. From the list of actions that drop down, select Send Back.
Send Back: Rejects the report. Approver is required to give a reason to send back report. The user/employee is notified by email and the report is moved to Inquiry state.
The employee will have to rectify the issues with the report under Inquiry and resubmit it once again.