What would you do if you wanted to work on quilt for someone you liked but had the material for the quilt he wants to sell?
My experience in other areas is: business deals and friendships/family do not meld. My niece bought a good running car from my hub and then wrecked the transmission and blamed the car. Bad feelings result. If he has connections to your family it could be bad if dispute arise.
I liked the previous idea given about make the quilt and let him sell it. Give him a commission for selling it. Pass the cost of fabric you bought on to the price of the quilt. After you sell a few, you will have fabric money and not need his money. You deserve more than 50% ?
I am wishing you luck when you decide what to do. What a nice dilemma.