Eveshka has a point - you're basically going to need to build the plush to make sure the pattern works.

There are a few options: Tell the client that they're welcome to buy the plush that you have to make along with the pattern. OR tell the client that you will be selling the plush of their character to recoup the time, materials, and effort put into it.

If they argue or hassle you, you're always able to say 'nope, sorry' and decline the project.