In chapter 20 of Neil Gaiman's American Gods, Hinzelman says that he saved Shadow's life because he owes a debt to Mr. Wednesday. Do we know why he owes this debt? Is it explained somewhere in the book?
There's nothing in the book to explain it.
Hinzelmann tipped his head on one side. He scratched his nose, thoughtfully, rocked back and forth as if he were thinking. “Well,” he said. “That’s a good question. I guess it’s because I owed a certain party a debt. And I’m good for my debts.”
“That’s the fellow.”
And that's all that's said about it.
Given that their personal histories go back nearly a millennium, it's quite likely that they've encountered each other repeatedly. Wednesday is, of course an inveterate hustler, a trader of information and collector of favours. It's not until later in the novel that we realise that he's been setting up a network of safe-houses to allow him to stash Shadow away from his enemies.
“Initially, simply the pleasure of your company. And I have information to share. What’s that phrase…oh yes. You may learn something to your advantage."
Their breath steamed in the night air. Christmas lights twinkled in the windows of the stores they passed. “It’s good of you, putting me up,” said Shadow. “I appreciate it.”
“We owe your employer a number of favors [Said Mr. Ibis]. And Lord knows, we have the room. It’s a big old house. There used to be more of us, you know. Now it’s just the three of us. You won’t be in the way.”