Step 1: Understanding the Question:
The question asks how Narshahwaj Khan fulfilled his promise or paid back his debt of gratitude to Amar Singh.
Step 2: Locating the Information in the Passage:
The latter part of the passage describes his actions after Amar Singh's death.
It states: "When Amar Singh was killed treacherously and his dead body was captured by Shah Jahan, Narshahwaj Khan came with his army and fought against the vast army of Shah Jahan. He with his entire army was killed in the battle."
Step 3: Final Answer:
Narshahwaj Khan returned the obligation by staying true to his word. After Amar Singh's death, he took his army and fought against the powerful army of Shah Jahan, sacrificing his own life and the lives of his soldiers in an act of ultimate gratitude and friendship.