´It was narrated that Abu Sa'eed Al-Khudri said:` “The Messenger of Allah (ﷺ) forbade selling what is in the wombs of cattle until they give birth, and selling what is in their udders unless it is measured out, and selling a slave who has fled, and selling spoils of war until it has been distributed, and selling Sadaqah until it has been received, and what a diver is going to bring up."
Related hadith on this topic
Explanation & Benefits
Maulana Ataullah Sajid
Benefit:
All these forms fall under bay‘ al-gharar (transaction involving uncertainty/deception). However, if the milk is purchased after being measured, then there remains no gharar (uncertainty), thus it is permissible.
Source: Commentary on Sunan Ibn Mājah by Mawlānā ‘Atā’ullāh Sājid, Page: 2196