Add a feature, that will allow to retrive an amount of transaction, that was not refunded yet.
I.e. original order with 5 items was made for 50$ and transaction was authorized and captured.
Customer returned 1 item and refunded 10$.
Need an API method, that will allow to retrive the amount, that can be refunded.
Right now there are 2 ways to solve this case:
1) POS side tracking. This methods requires huge amount of analytics on server side and may cause the data to be incorrect if system has multiple clients, that has their own storages.
2) Cashier tries different amounts to guess which one is available.
