In general terms, UBI seems sane. I see two implementation problems. First, worldwide society competition: Can my society support UBI and compete with China? Second, automatic system balancing. What is the correct UBI level? Can we leave it out to be hashed by politicians?
With a basic income, now most of these people have more money. So I can charge $3 per apple, and even if I only sell 4 of them I still make $2 more than I made before. I eat the last apple myself, and the person who only has the basic income doesn't get an apple at all.
I think universal welfare makes more sense.
Your $12 in case B has a purchasing power equivalent to $8 in case A.