Here's what happens:
* The Tin Woodman's Dilemma: The Tin Woodman is stuck in a deep ditch, unable to move. He can't even reach his oil can to keep his joints from rusting.
* The Queen of the Field Mice Steps In: The Queen of the Field Mice and her subjects, hearing his cries for help, come to his aid. They use their tiny teeth to gnaw away at the earth surrounding the Tin Woodman, allowing him to escape.
* The Tin Woodman's Gratitude: The Tin Woodman is incredibly grateful for the mice's help. He promises to never harm a mouse again and vows to be their protector.
No Repayment
The Tin Woodman doesn't repay the mice in any material way. The act of gratitude and his promise to protect them is considered repayment enough in the story.
Let me know if you have any other questions about *The Wonderful Wizard of Oz*!