If I was commissioned to do a portrait, I would sell the person my practice sketches and paint if they wanted them, I would just charge more. (Paint is expensive)

If I was building a collection for a show, I wouldn't. But personally commissioned work is different; you have to understand what your customer wants and accept that they're paying you for your skill, not your artistic sensibility.

All that said, I know plenty of people that wouldn't sell these things. My advice to OP is to find a different photographer.