The first is that he must be free; the opposite is a slave. A slave’s sale or purchase is not valid except with his master’s permission. The reason for this is that a slave owns nothing; whatever is in his possession belongs to his master. The evidence for this is the Prophet’s statement: “Whoever sells a slave who owns property, his property belongs to the seller, unless the buyer stipulates otherwise” (1). Thus, we say that a slave’s sale is not valid except with his master’s permission.
The second is that he must be mature; the opposite of a mature person is a minor.
For this reason, the author said: “The transaction of a minor or a spendthrift is not valid without the permission of a guardian”—even if he is an adolescent of fourteen years of age and is skilled and proficient in buying and selling, his sale is still invalid, because he is a minor who has not reached maturity.
The evidence for this is Allah’s saying:
“And test the orphans until they reach marriageable age. Then, if you perceive sound judgment in them, deliver to them their property.” [al-Nisa’ 4:6]
Thus, Allah stipulated two conditions for handing over their property: reaching marriageable age—that is, attaining maturity—and financial prudence.
The third is that he must be sane; the opposite is an insane person. An insane person’s transactions are invalid. This also includes one suffering from senile incoherence: his sale is invalid. If a man is afflicted by such incoherence—that is, he has become aged and senile—and comes to someone and says, “I am selling you my house and my car,” the sale is invalid, because he has lost his reason. One of the conditions for a person’s transactions to be valid is that he be sane.
The fourth is that he must be financially prudent. A financially prudent person is one who manages his wealth properly, such that he does not spend it on something prohibited or on something of no benefit—for example, selling something worth one hundred for ten, or buying something worth ten for one hundred. The point is that he manages his wealth properly, and the opposite of a financially prudent person