First: the basic principle governing transactions is permissibility.

Second: benefits may be exchanged for compensation. Since it is permissible to exchange them for compensation, it is likewise permissible to loan them. For example, I may say to a worker, “Work for me for such-and-such a wage.” This is work over which a valid contract may be concluded and for which compensation may be given. Thus, you work for me one day, and I work for you another day. A slight disparity does no harm, since one person’s services may differ from another’s, whereas a major disparity would not be accepted.

His statement, “except human beings”: human beings may validly be sold but may not validly be loaned. What is meant here is slaves. Thus, if someone owns a slave, it is unquestionably valid for him to sell him, but it is not valid for him to loan him out, because this is not customary; because it involves humiliating a Muslim; and because it may lead to temptation and corruption. One might fear that a person would borrow a slave woman, have intercourse with her for several days, and then return her. For when a borrower returns a loan, the lender is required to accept the very item loaned. This could result in someone borrowing a slave woman, having intercourse with her for several days, and then returning her to her owner.

Some scholars said that it is valid to loan a male slave to a male, meaning that a person may loan another person’s male slave, because there is no fear that he will do anything to him. Likewise, if a female slave is loaned to a female, there is no problem; and it is valid to loan a woman to her unmarriageable male relatives. However, this is problematic, because, Allah willing, it will come later in the chapter on manumission that whoever acquires a relative whom he is permanently prohibited from marrying thereby emancipates that relative.

In any case, the author’s exclusion of human beings has a basis. Thus it is said: human beings may validly be sold but may not validly be loaned.