Is it permissible to condition the oculist's payment upon a cure (al-bur')?

Primary text

Conditioning the oculist's fee upon a cure is permissible, but the contract must be structured as a *Ja'alah* (stipendiary contract for an unknown task), not an *Ijarah* (lease/hire contract). An *Ijarah* requires a specified duration or a precisely known work. The text cites the precedent of Abu Sa'id concerning the healing charm (*ruqyah*), where a condition for a cure was stipulated, which was a *Ja'alah*.