That's just equivalent to UBI, isn't it? If you pay out UBI and get the money for it from taxes, then there's an income level below which people net gain money and above which people net lose money.
It is UBI with a "clawback". Conservative (Friedman's NIT version) and left wing (called Guaranteed income) versions of UBI like to place an ultra high tax/clawback rate on the lowest income levels. It is same as UBI if lower tax brackets are not the first bracket after "personal UBI received is paid back in taxes"
A NIT of 50% up to $20k income is equivalent to UBI of $10k with 50% as the lowest tax bracket. Under both, you pay 0 net tax at exactly $20k income, and you get a $10k refund at 0 other income.
Either one is still a 50% marginal tax rate no matter the name. On every $ you earn you only keep 50cents.
This still doesn't make sense to me. The UBI clawback that starts small and grows with each bracket makes sense. At 0 earned you get 20,000. And it's not until you hit the poverty line that it starts gradually being taxed back. So a family of four would pay 5 or 10 percent back if they were in the 40k-50k bucket.
It seems to me you're not discussing a NIT which pays money to workers, but rather a national minimum wage through the tax system. In this case 10,000 dollars. An NIT doesn't need a clawback because it diminishes as you go up in tax brackets. A UBI uses it to remove administrative overhead from issuing it and to make it clear that every adult, employed or not, is eligible.
NIT is paid to workers and non workers alike. As is UBI. The maximum NIT refund you get is at 0 earned income. When you earn income, your refund is lowered. That starts at $1 of income. Even if it is called a negative tax, it is still a positive marginal tax rate that reduces your net income for every $ earned.
An NIT refund comes from the IRS, while UBI can come from IRS or another department. They are still highly related concepts. Other than the most famous NIT proposal has a 50% tax rate on the lower incomes, and frequenly left leaning politicians, instead of UBI propose Guaranteed Minimum Income, with tax rates of 50% to 100% on the lowest incomes.
Sensible UBI plans use normal tax rates with higher rates on upper incomes if needed.
Guaranteed minimum income plans are either a 100% tax, when literally, all get a minimum income of say $20k, if you earned less than $20k, you don't keep any of those earnings. Practical, still left of center plans do change this to a more modest 50% clawback rate similar to welfare/EI. The most famous NIT proposal had a 50% tax rate on the lowest income. That is the exact same as the flawed GMI plans.
That sounds like a great way to do a poverty trap when you could simply add 20k-reported income to their account. It's entirely unnecessary to the concept of an NIT.
If you mean 50% tax bracket for the poor, yes it is a poverty trap. UBI is an improvement over welfare and employment insurance because it doesn't trap people into not working due to high clawbacks. There is also administration and annoyance savings from not policing/applying for benefits.