Taking off a Kippa - Igrot Moshe Yoreh Deah 3:2

In a teshuva from 1974, Rav Moshe was asked about whether it would be permitted for a person to remove his kippa in a business situation if keeping it on would cost him financially.

Rav Moshe replied that according to virtually all poskim wearing a kippa is a מידת חסידות, and even though it has been widely accepted by Jews it nevertheless takes a back seat in the face of potential financial loss, which qualifies as אונס. The one possible difficulty is the view of the Taz, who raises the possibility that not wearing a kippa would be considered חוקות הגוים. However, Rav Moshe rejects this view based on the fact that nowadays, a man walking around with a bare head is considered de rigeur from a fashion standpoint and is not done for any ritual purposes. As such, a Jew walking around with a bare head would not be considered to be conforming to any specifically non-Jewish practice, and when money is on the line he should remove his kippa rather than risk his income.