His statement, “and capital whose earnings provide for his maintenance,” means that it must likewise be surplus to capital whose returns cover his living expenses. For example, a man may have one hundred thousand; he could purchase a slave for forty thousand, but the returns from the one hundred thousand barely suffice for him and his dependents. If he were to buy a slave with part of it, the profit would decrease, and consequently his sufficiency and maintenance would diminish. Should we then say: “You are required to buy a slave for forty thousand, even if your means of support are reduced”? The answer is no, because that would cause him harm.
His statement, “books of knowledge”: this is on condition that he needs them—for instance, someone who needs scholarly books on jurisprudence, hadith, Quranic exegesis, monotheism, or grammar. The point is that he needs them. As for books he does not need—for example, if he has scholarly books in a field he is not studying, such as books on mathematics when he has no intention of learning it, or books on geology when he has no intention of studying that field—then he must sell them. Likewise, if he has two copies of the same book and can do without one of them, he must sell it, because he has no need for it.
Similarly, if he has books that he rarely needs, and their value would enable him to emancipate a slave, he is required to sell them—especially if he lives in a city with a public library, so that, should that issue arise for him a year later, he can go to the library and research it.
His statement, “and the repayment of a debt”: this is among the most important matters. A person may have one hundred thousand, but owe eighty thousand; he must first settle the debt, because repaying a debt is obligatory and is a right owed to other people, whereas the expiation is a matter between him and his Lord. If anything remains after the debt, and he does not need it for the matters mentioned previously, he should purchase a slave with it; otherwise, he is not required to do so. Debt is among the most important matters; indeed, a person may be given zakah to repay his debt.