Feature Request: Improve Refund Flow for overpaid invoices

[petzsch]

As a merchant, I sometimes have the problem that invoices get overpaid by the customer. In such cases it would be a helpfull improvement to have the option to refund only the overpaid amount (maybe minus a configurable processing fee)

The screen where I would like to have this option would be here:

[dennisreimann]

@petzsch Would it be OK like that? If you need the "minus configurable processing fee" I think we should consolidate the options more, to enable that percentage setting for all options. (Also see this comment)

[petzsch]

Looks good to me.
I think I can do without the configurable fee, but it could be useful to some merchants.

Current onchain fees are killing some use cases right now.