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Note: this question is posed merely to seek mekorot behind the question, since the poskim of my hashgachah have clearly ruled on this, even if I've never seen the teshuvot.
As a mashgiach, I am occasionally required to certify an event on Shabbat. However, by the same token, I would lose my job and my credibility if I were a mechallel Shabbat. Further, mashgichim who hechsher events on Shabbat are financially compensated, even though the non-Shabbat prep is fairly limited, usually only filling out information on cards which are posted as temporary te'udot.
What is the halachic reasoning which permits a mashgiach, whose shemirat Shabbat is necessary for his credibility, to be compensated for hashgachah work he does on Shabbat ?