Transaction posting is the final step in rental transaction. Once posted, transaction cannot be altered. However, if you really need to alter, you can “Undo” the transaction using Manage Transaction screen. Select the transaction in Manage Transaction screen, double-click it and click the Undo button. This will bring the transaction back into unposted state. However, you have to manually delete the transaction invoice in Xero because Xero do not allow add-on programs to update/delete posted transactions. Once the transaction is Unposted, you can open the transaction again in Rental transaction, make changes and post to Xero again.
Alternately, once the transaction is posted to Xero, you can make the changes in Xero invoice if required. However, any change you make to Xero invoice will not be reflected in rental transaction as Xero does not talk to Viberent, but Viberent talks to Xero. Nobody other than Xero programmers can modify their system to talk to addon systems like Viberent. So if you make changes in Xero, they wont be reflected in addon systems such as Xero.