How to let a guy down easy?

My best guy friend likes me, he said so over texting. I told him I kind of liked him too, which is sort of true, but I really LOVE someone else. I told him if he asked me out I would say yes. I meant it at the time, but now I regret saying that because I realize I do not like him that way. I know he is going to ask me out. How do I tell him "no" gently when I already said yes without ruining our friendship?