Make contracts even when you are starting a freeware of an open-source project. I know some (sad) stories where a freeware project was about to turn into a sales vehicle but due conflicts between team member contributions this couldn’t be achieved.
Remember: if you start a new project then you must have written contract. Besides general project info, make sure there’s a part where you describe whether the contributed work can be used even if the team member leaves.
If you are in a situation where you don’t have contracts and want to start selling your game you have two options. You either negotiate the deal with the contributors or get new art/music/contribution from someone else. And learn that next time you will have those contracts done in advance.