I have a question about etiquette for gift giving?

A friend of mine has been helping me move. He didn't want to take money or anything, but I wanted to give something to show my appreciation. He was talking about preordering the new Harry Potter book, so I went and got it for him. I let him know that I had taken care of it (so he wouldn't do it as well). My problem is, I think he is under the impression that I just preordered it for him, but he will still be responsible to pay. I am in fact taking care of the money for it. I think it would be kind of rude to order something and make someone pay for it. Anyway, should I just let it go for now and then when it comes in he will know I paid, or should I say something? I think it seems kind of tacky to randomly say later that I am paying for it, but I also don't want him to think I'm rudely ordering things for him to pay for. Any input? Thanks.