Let us say one day I might want to hire you to do something for me. The other day you may want to hire me to do something for you. Please explain to me why we cannot just agree, how much we would pay each other, and then pay what agreed after the job completion? What is wrong with that?
By the way, your concept is not new. I experienced it firsthand; I lived where the system, as you propose, was implemented. It was in Poland, before the system transformation in 1989. The government was an almost exclusive employer, and there was a table of democratically distributed income. They did not have such a fancy name for it; they called it a pay-table. It brought the nation to bankruptcy.