Is the sale of impure fish guano (Sirsijin) permissible?
Chapter on Selling the Musarrah (Animal with milk retained in udder)
Al-Mughni
Book of Sales
Primary text
The sale of impure fish guano (Sirsijin) is not permissible. This position is held by Malik and Al-Shafi'i. The evidence is that this substance is agreed upon to be impure (najis), and what is impure cannot be sold, analogous to carrion (maytah). Furthermore, it is impure excrement (rajee'), and thus its sale is forbidden, similar to human excrement.
Supporting text
Abu Hanifa permitted its sale based on the practice of city dwellers who trade it for their crops without any objection, which is considered an established consensus (ijma'). This argument regarding consensus is refuted because true consensus requires agreement among all scholars, which is not established. Moreover, it is impure excrement, making its sale impermissible, like human excrement.