Once rejected you are able to amend the slip and then select the option to take this back to it's original state before anyone has authorised it, this is done by doing the below:
From the main menu select Slip Postings.
Select the REJECTED tab.
Find your posting and select AMEND.
Change the detail of the transaction to what you need and click SAVE.
This will bring a message up to confirm if you want to re-activate the slip select YES.
Once this is re-activated the slip will go back into the TO POST tab ready for the authorisation process again.