My friend is making a short film and I don't want to financially support it.
Do you have a reason for not financing it?
I don't want to make personal investments when it comes to my friends.
It's your own choice to make and many would agree with you in that mixing finances and investments with friendships is a recipe for disaster. As long as you are not leading your friend on or setting a false expectation, it is fair for you to make that choice.
I would rather not make any deals.
You logic is perfectly sound and you should do what you feel is right. You are probably avoiding quite a few future issues by not financing their film.