In my experience, most clients want to see a finished painting of the finished painting before they say yes to a commission. The only way I will do so is if they pay me for all the time and effort that will take. They rarely agree to do this.

What I do instead is "describe" what the painting is going to look like. I show a sample of my work so they can judge the quality of the draughtsmanship and painting style, and I ask them to trust my judgement. So far, I have had no complaints.