(Exodus 22:24) "Im you lend money to My people": R. Yishmael says: Every "im" (generally "if") in the Torah connotes optionality, with the exception of this and two others; (Leviticus 2:14) "And im ("when") you offer a meal-offering of first-fruits," where it is mandatory. You say it is mandatory, but perhaps it is optional? It is, therefore, written (Ibid. 14) "You shall bring the meal-offering of your first-fruits." It is mandatory and not optional.