Re: Can someone tell me why Venmo is a thing?
person A owes $18.23 but only has a twenty,
If they are such a tight-wad that when bill-splitting with friends (the only people I'd enter into such an arrangement with in the first place) that they get shitty if they don't get their $1.77 exactly in change, or aren't happy only getting $1 back, they can just fuck off and never bill split with someone again unless they bring the right change to cover such a situation. I frequently say to people collecting money for the bill if it's only a dollar or 2 change "I don't expect any change - maybe leave any extra as a tip".
person B owes $42.63 but only has $35 on them and would like to owe the rest to person C,
that's person B and C's problem, not the one collecting the bill. Oh, and what ever happened to "you can buy me a drink at the bar we are going to, or the next time we go out, to cover it".
meanwhile person D has sneakily gone to the loo and will never come back
Then either a) next time they can pay up front before we order, or (preferably) b) they aren't actually a friend and can just fuck right off and will never be invited along again. Besides, if they'd do that, what makes you think they'd pay by venmo or any other payment process (e.g. cash), ever?
If you are bill splitting with people who you can't trust to pay back directly (or balance up next time), or who will quibble about a couple dollars, or who you know the positions won't be reversed next time so averaged out over multiple meals/drinks over months or years of a friendship won't balance out or be worth it (i.e. they are a good friend and they are worse off than you so you don't mind covering them sometimes, or vice versa), then, frankly, you shouldn't be in bill-splitting situations with them at all.