I just want to say that it's usually not a good idea to work on projects without upfront payment. Don't do it, it's not safe. If the buyer doesn't want to adhere to good business practice there's a trust issue somewhere, that's not a good business relation.
And as for contractors, don't be that guy. Do your work, give that extra 10%. Sign the deal, don't break it. That's why we have legal contracts. If you don't sign anything, it's a bad business deal. Always ask for paper, even for small things.
And never, ever sign on to revenue share deals. Pay is never guaranteed with those deals. You know just as much as your buyer how much you'll sell and that's uncertain and unsafe. Don't do that.