Hi,
I don't think that it's possible. If you change the order on the backend no change is made to the payment. It has already been processed, so it's not possible to change it.
By default, the transactions are captured automatically. There is the possibility to only authorize it:
www.hikashop.com/en/forum/4-how-to/54213...cally-capturing.html
That way, you might be able to change the amount manually on authorize.net's merchant interface (you would have to ask them if and how that's possible) before capturing the transaction. But I think that you will only be able to capture a smaller amount than the amount that was authorized by the user during the checkout. So It might not even suits your needs.
