To expand on this, google.com is registered through MarkMonitor, which is a registrar. Google Domains is also a registrar. A registrar cannot sell a domain that is owned, and certainly not one that is owned by a client on another registrar! There was some error on the Google Domains side that indicated a domain was available for purchase that was in fact not available for purchase. That's it. The money was refunded when the error was reported. It's the only possible sane solution to the problem.

Your comment implies that the sale should have gone through anyway, which is nonsensical. Otherwise we could have situations where I steal foo.bar from you (which you have registered with, say, NameCheap) by buying it through, say, GoDaddy, which is currently experiencing a similar bug that incorrectly marks your domain as available.