If Zayd tells 'Amr, 'Attend so I can measure it for myself, then you measure it out,' and they do so, is the subsequent measuring by 'Amr valid?
Chapter on Selling Assets and Fruits
Al-Mughni
Book of Sales
Primary text
If Zayd measures it for himself and then 'Amr takes it using that measurement, there are two narrations regarding its validity. If Zayd leaves the food in the measuring container and gives it to 'Amr to empty it for himself, it is valid, and this constitutes valid possession because continuing the established measurement is equivalent to initiating it, and starting a new measurement yields no additional knowledge.
Supporting text
The companions of Al-Shafi'i hold this invalid due to the Prophet's prohibition against selling food until the measure (Sa'an) has run through it twice. They argue that the purchaser's possession while it remains in the measuring container fulfills this condition.