Firstly: The Meaning of 'The Cause of Allah'
Sabil linguistically: is a path or way, used both as masculine and feminine, though the latter is more common.
In the way of Allah: is general, and is used for any sincere act of worship, obligatory and voluntary, seeking Allah, exalted, and proximity to Him. If used unconditionally it most often refers to jihad. The specific usage became so dominant that seemed synonymous with it and created a sense that it can only refer to it as a usage.
Secondly: For the Cause of Allah as an Avenue of Expenditure for Charity
'For the cause of Allah' is one of the eight valid avenues of legal charity expenditure. Consensus has been quoted on this by: Ibn al-Mundhir, Ibn Hazm, Ibn Qudamah, and Sham al-Din Ibn Qudamah.
Thirdly: Where it is Spent
This share is spent on jihad in the way of Allah. This is by agreement of the four schools of jurisprudence, the position of the Zahiris, and the majority of scholars.
Fourthly: Poverty as a Condition for the Conquering Soldier
It is not a condition that the soldier upon whom this share is spent is poor. It is permissible to give to the wealthy among soldiers as well. This is the position of the majority: Hanafis, Malikis, Shafi`is, and the majority of scholars.
Fifthly: Purchasing Weapons
This share encompasses a conquering army and its weapons. This is explicitly mentioned by the Malikis and Shafi`is, it is a position among Hanbalis, chosen by Ibn Baz and Ibn `Uthaymin, and to its accordance was the verdict of the Islamic Fiqh Academy as part of the Organisation of Islamic Cooperation.
Sixthly: Hajj and `Umrah
It is not permissible to give charity from the share of the cause of Allah to those wishing to perform Hajj or `umrah. This is the position of the majority: Hanafis, Malikis, Shafi`is, and a narration from Ahmad.