Collateral - Tradução em português

Estou traduzindo um livro para o português e esbarrei nessa palavra “collateral” que teria o sentido de “garantia”, mas não faz sentido em português no contexto da história(por exemplo, na primeira frase ficaria algo como “colocou Mitchell como garantia, mas não vejo sentido nenhum). Alguém poderia ajudar?

Luis Marin wasn’t hard to find. They had found the traitor trying to sneak into a hotel where his mother had taken refuge. Marin gave up without a struggle. Harper (the leader) put Mitchell (the subordinate) on COLLATERAL immediately, and gave orders that Marin be taken to the outpost north of town...
... Sergei (another subordinate) was strapping Marin to a high-backed wooden chair in the last room on the left;...
... First things first. “Let me know when Mitchell gets here, ” Harper said, and the guards nodded...
... There was a soft rap at the door. Mitchell had arrived...
... Harper walked to the door, cracking it open. “Mitchell, step in here, please.” Mitchell nodded and followed him into the room, bouncing the gurgling, dark-eyed COLLATERAL on her hip. A little boy, about a year old, his mouth and chin speckled with chips of the candy skull he was slobbering to mush in one chubby fist. Marin’s youngest nephew, and namesake. The boy goggled at the room, his brow drawing up “Any problems?” Harper asked.“Playpen in the yard, they never saw me, ” Mitchell said, in a cooing, girlish voice as she nuzzled the worried boy’s neck. “Isn’t that right, baby? They never saw me, did they?”...
... “You’ll kill him anyway, ” Marin breathed.
“No, sir, I won’t, ” Harper said. “No reason. He’s too young to tell anyone what happened. I’ll let him go, if you’ll answer me. On my Oath.”...