Dear Bike shop owner,
Thank you for admitting the mistake. This is unfortunate but things happen. You are obviously a shop with values so I want to remain a customer here. I realize some of the "blame" for coming in here uninformed and for waiting so long. Instead of spending good money after bad, I propose we come to an agreement where you buy the bike back for some fair price, sell it used to someone else and I'll take that money as credit and buy the correct bike from you. I'll be out a little and you'll be out a little, you will look really good and I'll be a happy customer."