Claim the Firstborn

The mana value cap is the entire hinge. Threaten effects had always been priced around what they could steal: a full-toughness dragon costs more to borrow than a token, so Act of Treason and its kin pay three mana to lift anything off the table. This one refuses to negotiate on rate and instead narrows the target pool, buying a one-mana price with a hard mana-value-3-or-less restriction. That constraint makes the card a puzzle piece rather than a tempo swing: you are not borrowing a bomb, you are renting a small utility creature and deciding what you can strip from it before end of turn. The untap-and-haste rider is what makes the rental worth paying for. Haste lets you swing with the borrowed body immediately, and the untap lets you fire off its tap abilities the moment it changes hands, so you are renting the creature's actions, not just its presence in your battle line. Where the design turns genuinely mean is the sacrifice outlet: feed the borrowed creature to it before the loan expires and the control effect never has to end, because there is nothing left to return. Absent that, "until end of turn" means exactly that; a tapped mana dork or drained hatebear goes back home once the turn passes. Because this is a sorcery, every use is proactive; you take on your own turn, extract the value, and dodge the awkward window where the loan is still just a loan. It lives or dies entirely on whether the deck around it is built to keep what it takes.