Currently approvers can reject individual expense but only on the Report Page by selecting "Reject" from the drop down menu by clicking the three dots on the right of corresponding expense.
But when approvers view expense details one by one, there are only two buttons "Edit" and "Close" at the top of the expense detail screen.
Please add one more button of "Reject" for rejecting the expense being viewed.
I believe nearly in all cases, approvers need to view the details to make a Reject decision. So the best place to have that "Reject" button should be on the Expense Details screen.