He gets no brownie points (and, more to the point, gets, at best, the NIL amount previously negotiated) if he sticks it out at UNLV.
He doesn't really know the other guys at UNLV (he's a transfer, after all) and doesn't really have NFL prospects to damage if he leaves for a better deal.
If he has a deal lined up somewhere else that pays him more, it makes a lot of financial sense to take that deal.
I don't like it, but if I was a college student and this was my last chance to make life-changing money, I'd probably do the same. There are some very good teams with some very bad QBs this year (Notre Dame, A&M, Florida State all immediately come to mind) and even more programs with deep pocketed boosters. $1M wouldn't be unreasonable—and if that was on the table for me, I'd probably take it.
What I wouldn't do, is kick my prior school on the way out. Airing UNLV's dirty laundry is just a bad look. Twist your ankle/hurt your shoulder/withdraw for personal reasons, but don't bash your former program.