It seems you pretty much have the job, is that so? I usually make samples with my time and money, not theirs. It helps you a lot to know what to expect on the job so you are less likely to get "surprises", and helps give an understanding of time involved. Since you own that sample you can do with it as you choose, put it aside to display to future clients OR incorporate it into the job by "selling" it to the client if you do not want to keep it. It is nice to have some samples of work for people to see in person. Some people can no visualize from drawings and need to be able to see and touch so a library of different styles, joints, textures etc... is usually helpful. Besides when you get in the groove of making parts one more is no big deal.
I think some smiths may charge a non refundable deposit for extensive design work to cover things like this but I am not familiar.
Rob