“…Limits and permits..”
זֹאת תּוֹרַת הָעֹלָה הִוא הָעֹלָה עַל מוֹקְדָה עַל הַמִּזְבֵּחַ כָּל הַלַּיְלָה עַד הַבֹּקֶר – שמות ו, ב
After a sacrifice in the Temple is slaughtered and some of its blood sprinkled on the Altar, the kohanim burn the choice fats of the animal (and in some instances, all its fats and limbs) on the Altar. Though all of this should ideally take place during the daytime, the verse above teaches us that if the sacrificial parts of the animal were not burned during the day, the sacrifice is still valid as long the fats and limbs may be burned on the Altar anytime throughout the night, until dawn.
The Rambam rules, however, “in order to distance from inadvertent transgression, [i.e., the sacrificial parts of the animal not being burned in time,] our Sages declared that the fats and limbs of the burnt-offerings should only be offered on the fire of the altar until midnight” (Hilchos Maaseh Hakorbanos 4:2).
Now, though the Torah authorizes and instructs the rabbis to create “fences” to protect the Biblical laws from being transgressed, we find, however, that the Sages limited their “fences” only to areas that the Torah does not explicitly say otherwise (see Taz, Orach Chaim 588). In view of that, considering that the verse explicitly permits burning the sacrificial parts of the animal “all night until morning,” how could the Sages require (according to the Rambam) that everything be burned on the Altar before midnight?
One explanation for this is that the burning of the animal parts on the altar serves two purposes. Firstly, it is a component of the sacrificial service. In this capacity, like the other components of the service, the limbs and fats should be burned during the day. In addition, however, burning these parts of the animal also “prevents” them from being leftover until morning, at which point they would be disqualified from being offered on the Altar. When the fats and limbs are burned at night this second goal is achieved, but not the first.
Accordingly, the Rambam can explain that only in instances where the Torah states (explicitly) that a “positive” mitzvah may be fulfilled in a certain manner, the Sages are limited from “restricting” the fulfillment of the mitzvah. In instances like burning a sacrifice through the night, however, where the Torah’s permission is regarding an action that is only a preventive measure (but is no longer a positive mitzvah), the Sages may indeed “limit the permit” to further prevent these parts of the animal from being left until morning.
— Likkutei Sichos, vol. 3, pp. 949-950