You're investing in his business. You're putting up cash (the monthly fee of your software) in exchange for an ill-defined share of his future profits. It's t

You're investing in his business. You're putting up cash (the monthly fee of your software) in exchange for an ill-defined share of his future profits. It's the same as buying shares in his company. Why would you invest in his business? Don't you have your own business to run? Say this: "I've thought about your offer, and it just seems too complicated to get into. I think it's best to stay with a straight monthly fee. The best I can offer you is [5% off of whatever you offered him before]."