His statement, “for the equivalent of either of the two,” is connected to “an exchange”; that is, property—even if owed as a liability—or a benefit may be exchanged for the equivalent of either of the two. This means that a contract may concern three things:

specified property, property owed as a liability, and a benefit. If each is combined with each of the others—three multiplied by three—the result is nine possible forms:

The first: specified property for specified property.

The second: specified property for property owed as a liability.

The third: specified property for a benefit.

The fourth: property owed as a liability for specified property.

The fifth: property owed as a liability for property owed as a liability.

The sixth: property owed as a liability for a benefit.

The seventh: a benefit for specified property.

The eighth: a benefit for property owed as a liability.

The ninth: a benefit for a benefit.

It may be asked: since this is an exchange, why do we not make it six forms?

We say: because that which follows the preposition bi- is the price, whereas that upon which the verb falls is the item sold. Thus, if you say, “I sold you a book for one dirham,” the verb falls upon “a book,” so it is the item sold; while in our saying, “for one dirham,” the preposition bi- precedes “one dirham,” so it is the price.

The rule, then, is that what the verb falls upon is the item sold, and what follows the preposition bi- is the price.

His statement, “in perpetuity,” means that this exchange must be permanent, in order to exclude leasing. For leasing undoubtedly involves an exchange; thus, if you lease