The very existence of an intellectual property market is absurd. In my opinion, the scenarios in which person should expect to make a profit doing something creative are: A. The person is hired to create something. B. The person creates something that's not easy to copy. If you were hired to design a set for a play, you would get paid for your work. If you created a physical three-dimensional sculpture, someone else…
The person has produced the work and they offer the opportunity to enter into an agreement for payment after the fact.
You say you're fine with entering into an agreement before the work is produced, but you don't believe it should be possible to enter into such an agreement after the fact. This doesn't seem very consistent to me.