I read the tutorial from @emre on how to add a CC tip using the Add Calculation Payment Processor, it inspired me to try and adapt it to a Fast Cash payment button.
I set up a payment processor for my fast cash buttons and set it to confirmation.
It works like a charm except for one issue: if you press No during confirmation it just goes back to the ticket screen and does nothing. I am requesting that when you press No it would skip the Payment Processor and allow the transaction to go through showing Change Due.
Now I realize its not that easy because that could cause potential Credit Card fraud. So maybe a way to keep it separate from Credit Transactions… Maybe a Credit card Confirmation and a Cash Confirmation option. IF you select Cash confirmation option it will skip processor on No press. If you choose Credit Card confirmation it will behave as it currently does.
EDIT: I got it working through a series of Ask Question actions/Rules and Update Calculation Action. So this is not a priority but it would be nice to have the option to simply click a button on the Payment processor to disable the processor if No response is given. This would allow you to keep it working as is for Credit Transactions but if you decide to use it with cash it would move the transaction through without having to use Automation rules etc