well, yeah. if you aren't that bothered about the money, then that's one thing. in this case, if it's not, you're also fairly okay with it. you understand this might happen going in, and it's not the biggest deal in the world to you.
but if, at the end of the day, the money is VERY important to you, in the sense that you expect a certain amount or a set percentage, then you are shooting yourself in the foot by not drawing up a contract in advance. if someone promises you money and doesn't give it to you, and you think this was unfair, then you really have no options if you have no contract.