I have no problems doing people favors, but those are done for free with no expectation of payment. If I'm expecting to be paid, I want it in writing.
In my experience, good people don't ask for such favors.
Everyone who has asked or expected that from me has let me down. My relationship with the first and only person who talks compensation as a first course remains friendly and lucrative.
If you help a friend with a coding problem he's having with his startup, that's a favour. If you give up other paid employment to help him found a startup, that's business, and you need contracts and payment and all the things that go with business.
You should treat the favor as a gift, and not expect a quid pro quo. You shouldn't do more than you can afford as a gift for them; you shouldn't spend years of your life, or a large fraction of your salary, or anything of the sort. It's just a gift for a friend or family member.
That way, you aren't left hanging, expecting something from them, and having that poison your relationship. Be very careful about letting some kind of debt or obligation hang over the head of a relationship that you care about.
I am kind of involved in a college project. It's not about money but after working on their website for a month or so, these guys had trouble even giving a link back to my portfolio.
Some people really don't have conscience. I hope you get the payments.