To me it seems suspicious that they wouldn’t rather try to solve their problem with support and approached you, a developer. After all: This would be larger problem for them, as they would be unable to purchase (or donate on) any games from itch.io. It would have been in their own best interest to remedy the situation.

I’m not saying that it has to be what happened, but… Given some recent events, I would consider the possibility that they weren’t really interested in paying for the game in the first place and just wasted your time in the hopes for a free key or something…