What is the ruling on exchanging milk for colostrum (Lab'a)?
Chapter on Riba (Usury) and Exchange (Sarf)
Al-Mughni
Book of Sales
Primary text
If the colostrum has not been touched by fire, its exchange for milk is permissible equally, as it is considered milk exchanged for milk. If the colostrum has been touched by fire, the exchange is impermissible. This is because the application of heat solidifies its components and removes some moisture, thus preventing its exchange for the unprocessed milk, analogous to exchanging bread for dough or fried nuts for raw nuts. This aligns with the Shafi'i school's position.
Supporting text
The view held by the Qadi that an exchange is permissible even after fire contact is not correct because the binding of parts by heat and loss of moisture invalidates the parity.