Someone has approved a purchase that I'm not happy with, can I adjust this?
Once approved, it is not possible to reverse the approval and send the transaction back through the workflow. This is why it is important you have the right people making approvals.
However, if you spotted the issue before Payment, you have the opportunity to query it with the submitter and the approver. Where necessary you can open the transaction and delete it.
To help you better understand the Approvals module as a whole, please visit the module overview page here.