What debt of thanks does Amos believe he owes Violet?

The debt of thanks was for the love she gave him.

Amos nodded again. "We owe no one and there are coins in the kettle like a family of rabbits in a burrow." He looked at Violet and smiled the broad smile that meant more than any words. It was a debt of thanks he owed to her and would go on paying as long as he lived.