What is the ruling if an unknown or forbidden consideration, or an impossible consideration, is stipulated in a *Ja'alah* contract?

Primary text

If a specified consideration is unknown, such as stating, "If you return my slave, you shall have a garment," or "you shall have his saddle and gear," or if the consideration is forbidden, such as wine or a free person, or if it is something impossible to deliver, such as stating, "Whoever returns my slave shall have one-third of him," or "whoever returns my slave shall have one of them," and a person returns the slave, that person is entitled to the customary fee (*Ujrat al-Mithl*). This is because he performed an act for a consideration that was not delivered to him, thus earning the fair wage for his service, similar to the ruling in *Ijarah*.