Square Cash is absolutely brilliant. I hadn’t put much thought into how they are moving money without ACH.

It’s brilliant:

The genius of Square Cash is that someone realized you can process a refund without having an initial charge in the first place.  This makes debit card refunds an interesting corner case, and given that Square’s primary business is credit card processing, it is not surprising that they are the ones that figured out this was possible (and presumably get a pretty great deal on the rates while they are at it).