Is an agreement to build a wall for one thousand dirhams considered an Istisna' contract or an Ijara (lease) contract for work, and what is the difference between them?

If the worker (ajir) is the one who provides the necessary materials for building the wall, then it is an istisna' (manufacturing contract). However, if the owner of the wall is the one who provides the materials, and the worker only provides the labor, then it is an ijarah (leasing/hiring contract). Ijarah differs from istisna' in that, in ijarah, the object (or material) is from the hirer (musta'jir), and the work is from the worker (ajir), whereas in istisna', both the object and the work are from the manufacturer (sani'). Furthermore, the subject of sale in ijarah is the work, while in istisna' it is the described object (or item) held liable (in dhimma).