If Zayd gives 'Amr dirhams and instructs him, 'Buy with these the equivalent food that you owe me,' and 'Amr does so, is the transaction valid?
Chapter on Selling Assets and Fruits
Al-Mughni
Book of Sales
Primary text
If Zayd gives dirhams to 'Amr and instructs him to buy the equivalent food he owes him, and 'Amr complies, the transaction is invalid because Zayd's dirhams cannot serve as the consideration for 'Amr's debt to Zayd. If 'Amr buys the food with those dirhams, either specifically or on his liability, it is treated as the act of an unauthorized intermeddler (Fudhuli), as previously explained.
Supporting text
If Zayd instructs, 'Buy food for me with this, then take possession of it for yourself,' the purchase is valid, but the possession for himself is invalid, based on the ruling for similar scenarios. If Zayd only says, 'Take possession for yourself,' it is permissible. Ahmad explicitly stated this ruling for analogous issues. If the food is placed in 'Amr's possession for Zayd, and Zayd authorizes 'Amr to take possession from himself, it is valid according to our view. The companions of Al-Shafi'i say this is invalid because one cannot take possession for oneself from oneself.