The 87th mitzvah is that an animal that has been designated as a substitute [for a sanctified animal] itself becomes sanctified.1

The source of this commandment is G‑d's statement,2 "[If he replaces one animal with another,] both the original animal and its replacement shall be sanctified."

Our3 Sages said explicitly in tractate Temurah4 that G‑d's statement5 (exalted be He), "no substitutions may be made" is a lav she'nitak l'aseh6 (a prohibition with a remedial positive commandment): [after the statement is made that lashes are never given for a lav she'nitak l'aseh, the Gemara asks,] "But what about Temurah, which is a lav she'nitak l'aseh [and the punishment is still lashes]!"

There, the Gemara elaborates on the reason why Temurah is punished by lashes even though it is a lav she'nitak l'aseh: "One positive commandment cannot outweigh two prohibitions." This means that the prohibition of Temurah is said twice: [1] "one may not exchange it," [2] "nor offer a substitute for it."7 However, there is only one positive command: "both the original animal and its replacement shall be consecrated."

We have therefore explained what we set out to do [i.e. to prove that this counts as a positive mitzvah.]

The details of this mitzvah — when the substitution is binding and when it is not, what is its status, and how it is sacrificed — are explained in tractate Temurah.