The verses in question reproduced below are not part of a Ghazal, but rather a verse-set known as a qat'aa (if you have a diivaan you will find it at the very end after all the ghazals).
iftaar-e-saum kii jise kuch dast.gaah ho
us shakhs ko zaroor hai rozaa rakha kare
jis paas roza khol ke khaane ko kuch na ho
the one who has the wherewithal to open his fast
that person should indeed keep his fast
the one who has nothing to open his fast with
what else could he do but be constrained to "eat the fast."
[Translation: Anjum Altaf]
As with many Ghalib verses, this one too has layers which turn on paradoxes and word play. What is fasting? It is defined in relation to eating or the ability to eat. If one doesn't or can't eat, then where is the question of fasting. So not fasting need not be a sign of lack of faith. And then the use of "rozaa khaanaa" is great because it simultaneously suggests that such a person (poor, indigent) should not be required to keep rozaa (उसे रोजा खाने के अलावा क्या चारा है?) and that such as person may have nothing to "eat" except the rozaa itself. That is, the idiomatic use of the phrase "rozaa khaanaa" makes the verse witty. The phrase means not keeping a fast, but it can also be taken literally to mean eating the fast itself, that is staying hungry. So the person is both fasting and not fasting at the same time.
So much for Ghalib's paradoxes. I could spend the day exploring them! But we also chose this verse because it raises important question about hunger and choice today. For this I encourage you to visit The South Asian Idea Weblog and see the discussion there.