Six conditions must be fulfilled by the seller and the buyer [for the transaction to be valid]: 1. they must be of the age of legal responsibility (bāligh); 2. they must be sane (ʿāqil); 3. they must not be foolish with finances (safīh); i.e. they must not spend their wealth in futile ways; 4. they must have an intention to buy and sell. Therefore, if, for example, someone jokingly says, ‘I sell my property’, the transaction is void; 5.
they must not be compelled by anyone [to carry out the transaction]; 6. they must, respectively, be the owners of the commodity being sold and the payment made in exchange. The rulings about these conditions will be explained below. Ruling 2092.* A transaction carried out with a non-bāligh child who acts independently in the transaction is void even if it is carried out with the permission of his guardian.
The exception to this is a transaction of things that have little value and with which it is normal to transact with a non-bāligh child who is able to discern between right and wrong (mumayyiz); such a transaction is valid if the child has permission from his guardian. If the transaction is carried out with his guardian (walī) and the non-bāligh mumayyiz child only says the formula (ṣīghah)[1] for the transaction, it is valid in each case.
In fact, if the commodity or the money belongs to someone else and the child sells the commodity as the agent (wakīl) of the owner or buys something with the money, the apparent (ẓāhir)[2] ruling is that the transaction is valid even though the mumayyiz child may be independent in having disposal over the commodity/money.
Similarly, if the child merely acts as an intermediary for delivering the money to the seller, the transaction is valid even if the child is not mumayyiz because in reality, two bāligh people will have transacted with one another.