This is nonsensical for numerous reasons
1. The federal government doesn't have tax revenue. It issues a currency and then taxes go toward countering inflation. The notion that a currency-issuing government needs to "balance its budget" in its own currency is a political fiction. Also, policies that benefit people at the lower end of the income spectrum disproportionately actually move money around in the economy, which generally speaking is less inflationary than allowing it to accumulate in various silos like hedge funds
2. A "negative income tax" doesn't have to require employment. You can make zero dollars in income as a self-employed person, and the self-employed still track their income for taxation purposes
3. There are many unnecessary-to-malicious subsidies and inefficient welfare programs that could easily have their rationale subsumed by UBI, and doing so would save considerable cost that is mostly the overhead of all the means-testing personnel and in some cases (like SNAP) completely separate financial machinery necessary to maintain them
Even setting all that aside, you could make considerably more "tax revenue" by funding the IRS or reversing some of the nakedly corrupt corporate tax breaks that have been created over the last several decades. There always seems to be more money to subsidize artificially lowering the price of certain goods, fund incredibly inefficient private government contractors to do things that once cost considerably less for the government to do itself, fund drug research only to then allow the resulting breakthroughs to be patented by a private firm and then gouge people for treatment, or buy ludicrously expensive weapons for militarized police forces, but propose anything that actually benefits people and suddenly everyone's worried about the costs.
Not to put too fine a point on it, but all this "Well have you considered the cost?" handwringing people do when UBI comes up just drastically misunderstands how governments use money, while claiming to be "responsible" and "realistic"