In Amazing Spider-Man #39, a disguised Norman Osborne (the Green Goblin) hires a bunch of hit-men to take on Spider-Man and does it by giving them half the money now. Literally: he splits the bills in half.
Assuming that the hit-men got their money, they would tape the dollar bills back together. Would this still make the bills legal tender? Or is this something that would only work in fiction?
Assuming both halves have the same serial number printed on them, yes - a glued back together torn bill would be valid. You may exchange it at any US bank. If banks don't want to deal with that - send it to the Bureau of Engraving and Printing (BEP). If you only have one half, and it is exactly one half - then it is useless. That is why the person in the comics said that his halves are useless to him. The banks or the BEP will want at least most of the paper currency to replace it.
The act itself (tearing the physical currency intentionally) is a felony with up to 5 years in the Federal prison.
Under US law, if you clearly have more than half of a torn bill it is worth its full value; the smaller piece is worth nothing... except that having both halves makes the banking system much happier, since it prevents some particularly stupid counterfeiting attempts.
So this proposal wouldn't be cheat-proof unless the cut is close enough to the middle to make determining 51% difficult. And I'd like to see you try to explain to a bank how so many bills were cut in half...
(This is more normally an issue when money has been damaged unintentionally, of course.)
The question is about the US but to add the European perspective: The rule over here (I only know German law, but assume it's the same for all of the Euro area) is that you need more than half of the bill or you have to be able to prove that more than half of the bill was destroyed (good luck) in order to get it replaced. Deformed coins can also be replaced.
But all only as long as you didn't break it on purpose. So giving half of the bill to the cab driver would be on purpose and (if the central bank knows about it) make the bill (or coin) invalid.
German information: https://www.bundesbank.de/Navigation/DE/Aufgaben/Bargeld/Beschaedigtes_Geld/beschaedigtes_geld.html
As others have noted, US law says that if you have over half the bill, it's worth the full value, under half is worth nothing. I presume if it is very close to half, if even careful measurements show that you have 50.5%, you'll have difficulty cashing it in, precisely because the government and the banking system aren't going to allow themselves to be easily fooled by someone cutting bills in half and then trying to redeem both halves.
I've seen several comments on here about how you'd explain to the bank how so many bills were cut in half. What if you just told them the truth? Not the part about killing someone, of course, but tell them that you made a deal, neither of you wanted to bother with complex contracts and having to go to court if the other side didn't pay up, so your buddy cut all the bills in half, etc. As you now have both halves and they clearly have the same serial number, this no real evidence of fraud. Okay, this is technically illegal -- 18 US Code Section 333, "Whoever mutilates, CUTS, defaces, disfigures, or perforates, or unites or cements together, or does any other thing to any bank bill, draft, note, or other evidence of debt issued by any national banking association, or Federal Reserve bank, or the Federal Reserve System, with intent to render such bank bill, draft, note, or other evidence of debt unfit to be reissued, shall be fined under this title or imprisoned not more than six months, or both." But you didn't do it, the other guy did. I presume the point of this law is to say that you can't get a hold of currency belonging to someone else and mutilate it so as to make it worthless. As he's now given you both halves, I doubt anyone would bother to track him down and prosecute him.
Just BTW, while checking up on the details of the law, I stumbled across 18 USC 336, which says that it's illegal to write a check for less than $1, with penalties of 6 months in prison. I just got a check from AT&T for 15 cents for one of those class action suits where the lawyers get $100 million and the victims get 15 cents each. Apparently that was illegal.
