That's not a recourse. They can of course offer a full refund. However the OP is not obliged to accept. One does not walk away from a contract capriciously, just like that.

They need to find a way to offer what they sold, or better.

Of course, to the extent that the purchaser is willing to accept a full refund, plus some X$ as a way of compensation, everyone can be made happy.

But simple solution is to move the OP to J. Moving him back to Y is not acceptable. Again, they might try behaving like a bully. But the OP should not put up with it.