The Passage of Time and the Proceeds of Labor

Are there cases where it's impossible to distinguish the passage of time from the proceeds of labor? If you pay for someone's time, what you're really doing is obligating them not to do things that they would really be able to do otherwise. You can buy something that's at this point at the moment of the contract purely potential. But even if you are buying ultimately the product and not his labor and attention and effort, the product doesn't exist.