They paid you for the tickets, you owe them the tickets. What they decide to do with them is their concern, not yours. How can you be expected to give them back any money when, as you say, none of this is your doing?
How is this any different than if they had purchased and taken possession of their own tickets instead of utilizing the convenience of having you do it for them? Or, if you had delivered the tickets to them right away instead of holding them? In these last two cases there would seem to be no possibility of arguing that you somehow owe them a refund.