Commentary for Parah 11:8

מנגבו – in order that the fit/suitable water does not combine/mix with invalid water.

The hyssop that was used for sprinkling [the hatat waters] is also fit for cleansing the metzora. As we stated in yesterday's mishnah, hyssop was used in other purification rituals, including the purification of the metzora. If it had already been used for the hatat ritual, it may be used later for the purification of the metzora.

אע"פ שנגבו פסול – and even though it was not defiled, from when it became susceptible to receive ritual defilement.

If it was gathered for firewood, and liquid fell upon it, it may be dried and it becomes fit. Hyssop was gathered for firewood and it got wet. Because he gathered it for firewood, the liquid that came into contact with it does not cause it to be receptive to impurity. If one wants to use it for sprinkling he still can. He must first dry it off so that invalid waters don't mix with the valid waters drawn for the sprinkling.

לקטו לחטאת – to sprinkle upon it the waters of purification.

If it was gathered for food, and liquid fell upon it, even though it was dried, it is invalid. On the other hand, if he gathers it for food, it is susceptible to impurity when it becomes wet. This is a general rule with regard to food when food becomes wet, it becomes susceptible to impurity. Even if he dries it off, he will not be able to use it for sprinkling.

כמלקט לעצים – and he dries it, and it is fit/suitable. And such is the Halakha (according to Rabbis Yossi and Shimon).

If it was gathered for [the sprinkling of the waters of] the hatat, it is subject to the same law as if it were gathered for food, the words of Rabbi Meir. Rabbi Judah, Rabbi Yose and Rabbi Shimon ruled: as if it were gathered for firewood. The sages disagree with regard to the case where the hyssop was gathered specifically to be used with the hatat waters. Rabbi Meir says that in this case any liquid will cause it to be receptive to impurity and thereby invalidate it. The other rabbis (three of them note this is unusual) say that as long as he dries it off, he can still use it.