I don't know of an extension that will do this automatically from the front end of the site.
We have worked quite extensively with the Braintree system for stored payments (i.e. subscription payments) within Magento 2.
You can build a custom system that allows you to process refunds back onto a card from the admin side. But it's mostly a manual process, unless you want to build in some business logic to handle it.
That said, what you're asking to do has a pretty substantial risk if your site ever gets hacked or attacked. It would be pretty easy to get into the Magento admin using a mySQL attack tool, and give a particular user $5000 in store credit, and then they can automatically move it over to their account.
The other thing to keep in mind is that you're going to have to pay a transaction fee on the refund; this is usually about the same as the percentage that you'll pay for charging a credit card. I'd budget 3% or so for that.
I hope this helps!