Interesting math problem. I need a proper economist or mathematician who legitimately knows their ****.?

Well not really a math problem more of ethics, but you could easily buy the hoodies with out him and then sell two of them. One to the friend and another to whoever, giving you even more profit. The fact that your friend is having you get him one is between you two not the other guy who isn't involved. If you are all 3 friends then I would say you should each pay fair amounts or atleast split the $15 in to $7.50 between you and the original friend involved and keep the third party uninformed. If you are not all friends and your friend still wants part of the 15 then he's in no place to bargain, as I said you can do it without him. If you are generous give him 5 bucks and call it a day. Its $5 he shouldn't really care anyway.