That seems to be feedback extortion. Keep it polite, do what you have to do, and if he leaves bad feedback then show that exchange to ebay and ask to have the feedback removed.
OTOH, from the buyer's point of view it certainly appears that you are keeping him from winning this because the bids are low. Why not have your brother bid on it? You will end up paying 10% for your own item but you did make an error.