An equivalent way to describe Friedman's negative income tax proposal is that everyone pays their taxes as normal and then $20k/year (or however much) is given back to literally everyone, regardless of income.

I like that version because it makes more clear that you don't have perverse incentives with respect to working: you never get more money by working less. The $20k/year is guaranteed unconditionally and you can supplement it with a job if you choose.