Is there a way to know what modification an approver made?

It is not possible to see the modification done by an approver in Pressero Admin, because the system updates itself with the latest information. However, you can alternatively use New order email to compare the change made by the approver. Whenever an order is placed it generates the new order email with the information that user has chosen while placing an order. You can compare the information on this email with the order that is approved with modifications and you will be able to understand what changes were made on the order.  
By default the New Order email only goes to the user who placed an order, if you would like to receive this information then you can edit the New Order Email Notification and add yourself as a recipient of the New Order Email.