Katha-1-1-19-eṣa te'gnirnaciketaḥ

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

Oh Nachikêtas, this is the fire leading to heaven that you craved for, by the second boon; and the expression ‘the boon has been granted’ should be supplied by way of concluding what has been said again, people will call this fire by thy name; this is the fourth boon which I granted being delighted with thee; Oh Nachikêtas, demand the third boon. The meaning is that Death considered himself a debtor if that were not granted..

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda