Asking someone for money without a product to give them might seem counterintuitive but it’s something we do every day. The trick is coming up with acceptable terms.
When an order is placed at a fast food restaurant, you pay, and they immediately start making the food. The promise is that you’ll only have to wait a couple of minutes for the meal.
With Kickstarter you agree to pre-pay, but only if enough others also pay.
For your product and customer, what terms might be agreeable to them to pay you for a product when it doesn’t exist or is in early development? Consider:
All these options give value to your customer in the immediate or near future and this gives you feedback that the idea behind the product is valuable.
You can even de-risk the decision for your customer by offering things like money-back guarantees.