From: Rabbi Moshe Sofer
To: Unknown recipient
Why do Rishonim prefer slaughtering on Shabbos for a dangerously ill person over feeding him neveilah, yet Rabbi permits feeding untithed produce rather than tithing on Shabbos? The Chasam Sofer resolves this by distinguishing that tithing requires multiple sequential Shabbos violations while shechitah is a single act.
Source: Shu"t Chasam Sofer OC §79