In searching for just the right title- one that conveyed the essence of the story AND sounded legal-ish – I landed on Indemnity. An indemnity is either a payment for some wrong committed to remove the penalty OR it's a security against some damage or loss. Insurance, in way.
Before this book, I'd only ever heard the word "indemnity" in an insurance or legal context. You may be familiar with the Billy Wilder film from the 1940s, Double Indemnity about an insurance fraud scheme. This fall, reading Beowulf for school, I found that the king paid the warrior an indemnity when one of Beowulf's men was killed. The king felt responsible, so he paid.
Payment for a wrong committed… Insurance, or protection, against future damages. That's Jack.
Jack is the indemnity.
Do you think the title fits?