I'm not going to let a friend stay with me even though I already said they could.
Why won't you let them stay with you then?
It was always just supposed to be a joke.
Unless your friend knows you were joking about having them stay, it's really rude to change your mind about something like that. At minimum, you should help them find another place to stay.
I hate having to be helpful.
Well you shouldn't have lied to him about being able to stay with you then. You should let him stay with you because that's what you promised.